433 8C06 IUPZAHYZLTJ5FVC5ASSTC7 Nieznany

○

Routing and
Switching Alogrithms

Terms you’ll need to understand:

✓ Distance vector protocols

✓ Routing Information Protocol (RIP)

✓ Interior Gateway Routing Protocol (IGRP)

✓ Enhanced Interior Gateway Routing Protocol (EIGRP)

✓ Link-state protocols

✓ Intermediate System to Intermediate System (ISIS)

✓ Open Shortest Path First (OSPF)

✓ Hello packets

✓ Border Gateway Protocol (BGP)

✓ Interior Border Gateway Protocol (IBGP)

✓ Exterior Border Gateway Protocol (EBGP)

✓ Summarization

✓ Multicast

Techniques you’ll need to master:

✓ Describing the mechanics of RIP, IGRP, EIGRP, ISIS,

OSPF, and BGP

✓ Setting up IP routing protocols in a simple design

scenario

✓ Identifying the IP routing table for each routing protocol

○

Chapter 6

This chapter presents the commands you need to know when configuring the
various IP routing protocols on Cisco routers. Frequently, the terminology pre-
sents the most challenging aspect of understanding routing with Cisco IOS.
Therefore, after reviewing the basic CCIE blueprint objectives covered in this
chapter, we’ll offer a brief overview of basic terminology. Then, we’ll move on to
discuss the Cisco IOS routing configuration commands.

The following CCIE blueprint objectives as laid out by the Cisco Systems CCIE
program are covered in this chapter:

➤ Border Gateway Protocol (BGP)—Peer groups, route reflectors, confederations,

clusters, attributes, autonomous systems (AS), route maps, filters, neighbors,
decision algorithm, Interior Border Gateway Protocol (IBGP), Exterior Bor-
der Gateway Protocol (EBGP)

➤ Enhanced Interior Gateway Routing Protocol (EIGRP)—Metrics, mechanics,

and design

➤ Intermediate System to Intermediate System (ISIS)—Metrics, mechanics, and

design

➤ Open Shortest Path First (OSPF)—Areas, virtual links, stub, not so stubby ar-

eas (NSSA), area border router (ABR), autonomous system boundary router
(ASBR) redistributions, media dependencies, external versus internal, sum-
marization, designated router (DR), backup designated router (BDR), adja-
cencies, link-state advertisement (LSA) types, link-state database, shortest
path first (SPF) algorithm, authentication

➤ Routing Information Protocol (RIP) and RIP II—Metrics, mechanics, and

design

➤ Multicast—Design, protocol independent multicast (PIM), Distance Vector

Multicast Routing Protocol (DVMRP), Internet Group Management Pro-
tocol (IGMP)

As in other chapters in this book, additional information is provided in this chapter
for completeness and in preparation for additional subjects as the CCIE program
expands. By now, you should be aware that a CCIE candidate’s knowledge must
span a wide range of topics. One topic of particular emphasis involves under-
standing the Network layer and how Cisco devices can be used to accomplish
intelligent routing. This chapter, the largest chapter in the book, addresses these
focal-point topics to help you fully prepare for the CCIE exam.

○

Routing and Switching Algorithms

Summary of Available IP
Routing Protocols

Cisco IOS supports a number of IP routing protocols. Listing 6.1 shows the
routing protocols supported by Cisco routers. According to the CCIE exam objec-
tives, you’re only required to understand RIP, IGRP, EIGRP, ISIS, OSPF, and
BGP.

Listing 6.1

The router ? command.

R2(config)#router ?

bgp Border Gateway Protocol (BGP)

egp Exterior Gateway Protocol (EGP)

eigrp Enhanced Interior Gateway Routing Protocol (EIGRP)

igrp Interior Gateway Routing Protocol (IGRP)

isis ISO ISIS

iso-igrp IGRP for OSI networks

mobile Mobile routes

odr On-Demand Stub Routes

ospf Open Shortest Path First (OSPF)

rip Routing Information Protocol (RIP)

static Static routes

RIP, IGRP, EIGRP, OSPF, and BGP are called dynamic protocols because they
employ techniques that “automatically” discover and learn IP routing informa-
tion from other routers. Dynamic protocols use dynamic routing, a routing method
in which routers learn about IP networks without static configuration.

In the upcoming sections, we’ll cover the distance vector protocols—RIP, RIP II,
and IGRP. Next, we’ll look at a hybrid distance vector and link-state protocol—
EIGRP. Then, we’ll cover the link-state protocols—ISIS and OSPF. Finally, we’ll
discuss BGP, a path vector protocol, which is an advanced routing protocol used
extensively in the Internet.

Routing Information Protocol (RIP) I and II

Routing Information Protocol (RIP) is a distance vector protocol, which uses
hop counts as the metric. This metric determines whether an IP network will be
placed in the routing table. There are two versions of RIP—RIP I and RIP II.

Both RIP versions I and II operate over UDP using port 520. As discussed in
Chapter 2, RIP uses holddowns, split horizon, and poison reverse to avoid rout-
ing loops. RIP version I cannot carry subnet information, and it applies the de-
fault network mask to all networks. Hence, all networks within a RIP network
must have the same subnet mask throughout. RIP II does carry subnet informa-
tion, because RIP II can use variable length subnet masking (VLSM).

○

Chapter 6

RIP version I characteristics can be summarized as follows:

➤ Distance vector protocol

➤ Hop count is 15

➤ Broadcasts full routing table every 30 seconds

➤ RIP can load balance if the hop count is the same

RIP version II characteristics can be summarized as follows:

➤ Distance vector protocol

➤ Hop count is 15

➤ Supports VLSM (carries subnet information in updates)

➤ Authentication of IP routing updates

The deficiencies encountered in RIP I, such as no support for variable length sub-
net masks, spurred the release of RIP version II. RIP II’s biggest improvement over
RIP I is that RIP II supports variable length subnetting and authentication of
routing updates. RIP II is also a classless routing protocol, whereas RIP Version
I is classful. RIP II still has a hop count limit of 15. Furthermore, RIP II supports
multicast updates. While RIP II provides advancements over RIP I, it still con-
tains some of the deficiencies found in RIP I. For example, RIP II continues to
send full routing updates every 30 seconds and limits hop counts to 15 hops.

Cisco routers support both RIP I and RIP II. By default a Cisco router runs RIP
version I. Therefore, by default, the router will listen to RIP II updates but will
only forward RIP I updates unless configured otherwise.

Note: In order to configure RIP version II, you will have to be in the “router rip”
configuration mode.

To further clarify the operation of RIP I and RIP, let’s look at a sample RIP
configuration on a small network consisting of four Cisco routers.

Configurating RIP I and II

RIP is easy to configure, you start RIP with the IOS command router rip and
then apply the networks you want to be advertised. The IOS command to enable
the local network within RIP is:

network <classful networks to be advertised>

For illustrative purposes, let’s configure the network shown in Figure 6.1 for IP
RIP version I. Then, we can modify the configuration to enable RIP II.

○

Routing and Switching Algorithms

In order to get our RIP network operating, a few steps will need to occur so let’s
briefly discuss them. First, we will need to start the RIP routing process on each
router and then apply the networks that are advertised as shown previously. The
RIP configuration on all three routers is identical because we are using the classful
Class A address of 10.0.0.0.

Listing 6.2 provides the configuration for router R1 shown in Figure 6.1. Router R1’s
RIP configuration is identical to the configuration of routers R2 through R4.

Listing 6.2

IP RIP configuration.

hostname R1

....

router rip

network 10.0.0.0

10.1.1.0/24

10.1.3.0/24

10.1.2.0/24

10.1.5.0/24

10.1.6.0/24

10.1.9.0/24

10.1.4.0/24

10.1.8.0/24

10.1.7.0/24

Token

Ring

RIP example

Router rip
network 10.0.0.0

IGRP example

Router igrp1
network 10.0.0.0

EIGRP example

Router eigrp1
network 10.0.0.0

Configuration of all
routers is the same

Figure 6.1

RIP network scenario.

○

Chapter 6

As you can see in Listing 6.2, the network uses the Class A network of 10.0.0.0.
Hence, we only need one line to identify the directly connected networks. To
view the IP routing table, you type the IOS command show ip route. Listing 6.3
shows the R1 router’s routing table.

Listing 6.3

The show ip route command.

R1#sh ip route

.....

10.0.0.0/8 is variably subnetted, 16 subnets, 2 masks

R 10.1.9.0/24 [120/4] via 10.1.4.1, 00:00:22, Serial3

R 10.1.8.0/30 [120/1] via 10.1.4.1, 00:00:22, Serial3

[120/1] via 10.1.2.2, 00:00:14, Serial2

R 10.1.8.0/24 [120/4] via 10.1.2.2, 00:00:14, Serial2

R 10.1.3.0/24 [120/5] via 10.1.4.1, 00:00:22, Serial3

R 10.1.3.0/24 [120/1] via 10.1.2.2, 00:00:14, Serial2

C 10.1.2.0/24 is directly connected, Serial2

C 10.1.1.0/24 is directly connected, Ethernet0

R 10.1.7.0/24 [120/2] via 10.1.2.2, 00:00:15, Serial2

C 10.1.7.0/24 is directly connected, Serial1

R 10.1.6.0/24 [120/4] via 10.1.4.1, 00:00:22, Serial3

R 10.1.6.0/24 [120/1] via 10.1.7.2, 00:00:13, Serial1

R 10.1.5.0/24 [120/1] via 10.1.7.2, 00:00:15, Serial1

[120/1] via 10.1.2.2, 00:00:16, Serial2

C 10.1.4.0/42 is directly connected, Serial3

R* 0.0.0.0/0 [120/1] via 10.1.4.1, 00:00:23, Serial3

Let’s examine the table shown in Listing 6.3. Each R on the left-hand side indi-
cates an entire RIP network has been learned. The C shows that the indicated
network is directly “connected” to the router displaying the routing table. The IP
network is then listed with an administrative distance (a measure of trustworthi-
ness; the lower the better) and the hop count. For example, the network 10.1.5.0/
24 is indicated by the R on the left side. Because the hop count 1 is sent via Serial
1 and Serial 2, the Cisco router will load balance across the two paths to the
remote network 10.1.5.0/30. Following this is the next hop address (10.1.7.2 or
10.1.2.2, for example, to the remote network 10.1.5.0/30) and how long the net-
work has been valid in minutes and seconds. Let’s take another remote network,
say 10.1.9.0/24, in the routing table:

R 10.1.9.0/24 [120/4] via 10.1.4.1, 00:00:22, Serial3

We’ll define the network as follows:

➤ R—The network was learned via RIP (The key is shown at the beginning of

the table.)

➤ 10.1.9.0/24—Which subnet is learned and how many bits of subnetting are

applied

○

Routing and Switching Algorithms

➤ [120/4]—Administrative distance/hops

➤ via 10.1.4.1—The IP address of the interface that the router learned the route

from

➤ 00:00:22—How long ago was the route learned

➤ Serial3

—What interface the network has been discovered from.

As you can see in Listing 6.3, the network 10.1.5.0/24 is reachable via routers R2
and R3 with the same hop count (1) so RIP will load balance to this network
because of the multiple paths. Listing 6.3 also shows a default route. The default
route is used if there is an IP packet to an unknown destination. In Listing 6.3,
the default route will be sent to the next hop address 10.1.4.1 or router R4. Let’s
assume you have been supplied a default router from the source address
131.108.1.100. To create a default route in RIP, you use the following command:

ip route 0.0.0.0 0.0.0.0 131.108.1.100

Note: The combination of a source address of 0.0.0.0 and mask of 0.0.0.0 indicates a
special route known as a default route.

The preceding command injects a default route into any neighboring RIP rout-
ers. To view the characteristics of how RIP is operating on a Cisco router, you can
use the show ip protocol command, as illustrated in Listing 6.4.

Listing 6.4

The show ip protocol command.

R1#show ip protocol

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 21 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip

Default version control: send version 1, receive any version

Interface Send Recv Key-chain

Ethernet0 1 1 2

Serial0 1 1 2

Serial1 1 1 2

Serial2 1 1 2

Serial3 1 1 2

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.8.1 120 00:00:13

Distance: (default is 120)

○

Chapter 6

As you can see in Listing 6.4, the router sends updates every 30 seconds, and the
next expected update will be in 21 seconds; RIP version I packets are being sent
out on interfaces E0, S0, S1, S2, and S3; and the router is listening to both RIP I
and RIP II. The administrative distance, which is defined as a group of hosts or
routers under a common management, for RIP is 120.

For further study, let’s now change the subnet mask on all the serial links to a 30-
bit mask or 255.255.255.252. RIP I will not support variable length subnet masking
(VLSM), so we’ll turn on RIP II globally. To activate RIP II, you enter the com-
mand displayed in Listing 6.5.

Note: All serial links in Figure 6.1 have been configured with 30 bit mask. For
example, the link between R1 and R4 that was assigned the subnet 10.1.4.0/24 has
now become 10.1.4.0/30, and so forth.

Listing 6.5

Enabling RIP II.

router rip

version 2

The commands in Listing 6.5 will force the router to send and receive RIP ver-
sion II packets only. Let’s take another look at the routing table for R1 now that
RIP II is enabled for the router. Listing 6.6 shows the updated routing table.

Listing 6.6

The show ip route command after enabling RIP II.

R1#sh ip route

Gateway of last resort is 10.1.4.1 to network 0.0.0.0

10.0.0.0/8 is variably subnetted, 16 subnets, 2 masks

R 10.1.9.0/30 [120/4] via 10.1.4.1, 00:00:22, Serial3

R 10.1.9.0/24 [120/1] via 10.1.4.1, 00:00:22, Serial3

R 10.1.8.0/30 [120/1] via 10.1.4.1, 00:00:22, Serial3

[120/1] via 10.1.2.2, 00:00:14, Serial2

R 10.1.8.0/24 [120/4] via 10.1.2.2, 00:00:14, Serial2

R 10.1.3.0/30 [120/5] via 10.1.4.1, 00:00:22, Serial3

R 10.1.3.0/24 [120/1] via 10.1.2.2, 00:00:14, Serial2

C 10.1.2.0/30 is directly connected, Serial2

C 10.1.1.0/24 is directly connected, Ethernet0

R 10.1.7.0/24 [120/2] via 10.1.2.2, 00:00:15, Serial2

C 10.1.7.0/30 is directly connected, Serial1

R 10.1.6.0/30 [120/4] via 10.1.4.1, 00:00:22, Serial3

R 10.1.6.0/24 [120/1] via 10.1.7.2, 00:00:13, Serial1

R 10.1.5.0/30 [120/1] via 10.1.7.2, 00:00:15, Serial1

[120/1] via 10.1.2.2, 00:00:16, Serial2

R 10.1.5.0/24 [120/4] via 10.1.2.2, 00:00:16, Serial2

C 10.1.4.0/30 is directly connected, Serial3

R* 0.0.0.0/0 [120/1] via 10.1.4.1, 00:00:23, Serial3

○

Routing and Switching Algorithms

Notice that the serial networks display with the new 30-bit notation, and the
Class C networks display. RIP I would not be able to cope with IP networks with
varying masks.

Now, take a look at Listing 6.7, which shows the output you receive when the
show ip protocol command is issued after RIP II is in use.

Listing 6.7

The show ip protocol command after enabling RIP II.

R1>sh ip protocol

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 7 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip

Default version control: send version 2, receive version 2

Interface Send Recv Key-chain

Ethernet0 2 2

Serial0 2 2

Serial1 2 2

Serial2 2 2

Serial3 2 2

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.2.2 120 00:00:22

10.1.4.2 120 00:18:44

10.1.4.1 120 00:00:21

10.1.7.2 120 00:00:24

Distance: (default is 120)

In Listing 6.7, you can see that now only RIP II routing updates are sent and
received, and updates are still sent out every 30 seconds. Table 6.1 provides a
summary of common RIP I and II configuration commands.

Table 6.1

Summary of RIP commands.

Command

Description

debug ip rip events

Outputs IP RIP events, such as updates every
30 seconds

debug ip rip

Displays the RIP routing transactions

neighbor <ip address>

Establishes a link to a remote router via unicast

network <network number>

Runs RIP on the associated interface

(continued)

○

Chapter 6

Note: RIP is relatively easy to configure and troubleshoot when compared to other
protocols, such as OSPF. Therefore, RIP was popular in the early days of IP
networking.

Let’s now discuss another distance vector routing protocol—Interior Gateway
Routing Protocol (IGRP).

Interior Gateway Routing Protocol
(IGRP)

Cisco Systems developed a proprietary distance vector routing protocol, called the
Interior Gateway Routing Protocol (IGRP), in the 1980s in response to the major
limitations of RIP. RIP’s major limitation, of course, is its hop count limit of 15
hops. Cisco developed IGRP with a maximum hop count of 255 and an update
interval of 90 seconds while retaining most of the other characteristics of RIP.

The major disadvantage of IGRP is that it only operates on Cisco routers. In
contrast, IP RIP is an international standard and operates on any compliant host
device. IGRP, which only works with Cisco routers, increases the network diam-
eter by allowing a greater hop count limit of 255 hops. The default hop count is
100 hops.

IGRP uses a concept called autonomous systems (AS). An AS is a domain under
the same administration. The AS number will be unique in each domain, and the
numbers are controlled by IANA (www.iana.org). IGRP AS’s numbers are not

Table 6.1

Summary of RIP commands

(continued).

Command

Description

offset-list <access list> {in|out}

Modifies an incoming or outgoing hop count

<offset>

passive-interface <interface>

Stops RIP updates from being sent out only

timers basic <update>

Modifies RIP timers

<invalid> <holdown> <flush>

ip rip authentication key-chain

Specifies authentication parameters

<key chain>

ip rip authentication mode

Indicates the RIP mode for password authentication;
md5 or clear text is supported

ip rip send version [1] [2]

Specifies the version of RIP to send out to an
individual interface

ip rip receive version [1] [2]

Specifies the version of RIP to receive out of an
individual interface

○

Routing and Switching Algorithms

maintained by IANA but BGP AS numbers are. For example, to enable IGRP in
AS 10, you would use the following command:

router igrp 10

A router in the same domain would need to have the same AS number configured
in order for it to exchange routing information with other routers running IGRP.

IGRP’s key points can be summarized as follows:

➤ It is scalable to large networks, because it supports up to 255 hops.

➤ The use of a composite metric ensures that high-bandwidth links are used.

➤ It can load share up to six paths (the default is four paths).

➤ It is a distance vector protocol, which sends out updates periodically.

The metric used by IGRP to measure the best path to a remote network is to use
a composite metric that includes the bandwidth, reliability, delay, and load. By
default, only the bandwidth and the delay is used to measure how long a packet
will take to leave an interface. The bandwidth is calculated in kilobits, and the
delay is calculated in microseconds. The formula to calculate the IGRP metric in
its simple form is:

IGRP

metric

= 10

/Bandwidth + Delay/10

For example, assuming a delay of 20,000 msec on a 1,544K line, the IGRP metric
would be calculated as:

IGRP

metric

= 10

/1544 + 20000/10 = 6476+2000=8476

The full IGRP metric can be based on reliability and load (default metrics). If
these metrics are used, then the formula becomes:

IGRP

metric

= K1 * bandwidth + (K2 * bandwidth)/(256-load) + K3*Delay

In the preceding formula, the values K1 through K5 are constants. If the defaults
are not modified by the administrator, then K1=K3=1 and K2=K4=K5=0.

If K5 is not 0 or it’s given an appropriate value as directed by a qualified Cisco
engineer, then you also have the formula IGRP

metric

= Metric * [K5/(reliability +

K4)]. In effect, the metric is simply multiplied by the constant K5/K4. These
values should only be changed under the guidance of a qualified engineer at Cisco
systems. This metric was designed to ensure that networks are selected based on
a number of key parameters, and that packets are sent over the best path (unlike

○

Chapter 6

RIP, which would not recognize the difference between a 1,544Kbps line and a
64K line).

Let’s examine how to configure IGRP on a Cisco router using the same network
topology shown earlier in Figure 6.1.

Configuration of IGRP

For this section, let’s revisit Figure 6.1 and use IGRP instead of RIP, with AS
number 1. The configuration command required on all four routers is:

router igrp 1

network 10.0.0.0

Listing 6.8 displays router R1’s routing table.

Listing 6.8

The show ip route command after configuring IGRP on R1.

R1#sh ip route

10.0.0.0/24 is subnetted, 9 subnets

C 10.1.9.0 is directly connected, Ethernet0

C 10.1.8.0 is directly connected, Serial0

I 10.1.3.0 [100/158350] via 10.1.8.2, 00:00:01, Serial0

I 10.1.2.0 [100/160250] via 10.1.4.2, 00:00:01, Serial2

[100/160250] via 10.1.8.2, 00:00:01, Serial0

I 10.1.1.0 [100/89056] via 10.1.4.2, 00:00:01, Serial2

I 10.1.7.0 [100/160250] via 10.1.4.2, 00:00:01, Serial2

I 10.1.6.0 [100/160350] via 10.1.4.2, 00:00:01, Serial2

[100/160350] via 10.1.8.2, 00:00:01, Serial0

I 10.1.5.0 [100/160250] via 10.1.8.2, 00:00:01, Serial0

C 10.1.4.0 is directly connected, Serial2

Notice that the networks are now reachable via I (IGRP), and the administrative
distance is 100. The metric is also a larger number. IGRP will load balance if the
composite metric is the same. In the entry for 10.1.6.0/24, you can see that there
are two alternate paths via Serial2 and Serial 0, because the metric is the same
(160350).

To display the characteristics of IGRP, issue the show ip protocols IOS com-
mand, as shown in Listing 6.9.

Listing 6.9

The show ip protocols command after enabling IGRP.

R1#sh ip protocols

Routing Protocol is "igrp 1"

Sending updates every 90 seconds, next due in 18 seconds

Invalid after 270 seconds, hold down 280, flushed after 630

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

○

Routing and Switching Algorithms

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

IGRP maximum hopcount 100

IGRP maximum metric variance 1

Redistributing: igrp 1

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.2.2 100 00:00:18

10.1.7.2 100 00:00:53

10.1.4.1 100 00:00:57

Distance: (default is 100)

You can determine from the output in Listing 6.9 that the AS is 1, the update
interval is 90 seconds, and the default hop count is 100. Notice the administra-
tive distance (AD) for IGRP is 100. IGRP will be selected over RIP, because the
AD is lower.

You can force IGRP to load balance over unequal cost paths by using the vari-
ance command. Further, you can define how undesirable an alternate path can be
before that path is rejected. Keep in mind that IGRP does not carry subnet mask
information, so we cannot use networks with different classes of address (in other
words, IGRP is a classful routing protocol).

Table 6.2 provides a summary of the common IGRP configuration commands
that you will need to know.

The maximum hop count for IGRP is 255 hops, and the update interval
is 90 seconds. An IGRP packet can carry 104 networks.

Eventually, Cisco developed another proprietary protocol that improved on
IGRP—Enhanced Interior Gateway Routing Protocol (EIGRP).

Table 6.2

Summary of IGRP commands.

Command

Description

maximum-paths

Sets the maximum path; the default is 4

metric maximum-hops <hops>

Specifies the maximum hops IGRP will use; the
default is 100, and the range is from 1 through 255

neighbor <ip address>

Establishes a link to a remote router using a unicast

address

network <network number>

Runs IGRP on the associated interface

(continued)

○

Chapter 6

Enhanced Interior Gateway Routing
Protocol (EIGRP)

Cisco Systems followed the development of IGRP with Enhanced IGRP
(EIGRP). Enhanced IGRP combined the characteristics of distance vector pro-
tocols and link-state protocols (link-state protocols are addressed later in this
chapter). Therefore, EIGRP is commonly referred to as a hybrid routing protocol.

EIGRP uses distance vector properties to determine the best path to a network
and uses link-state properties when changes occur or when detecting new neigh-
bors. EIGRP uses the Diffusing Update Algorithm (DUAL), which provides for
fast convergence, VLSM, and partial updates. EIGRP supports other desktop
protocols, such as IPX and AppleTalk, which are discussed in Chapter 7.

The main characteristics of EIGRP include the following:

➤ Uses the same composite metric as IGRP, but the metric is multiplied by 256.

➤ Sends network changes, but does not send periodic updates.

➤ Load shares up to six paths. (The default is four paths.)

➤ Serves as a hybrid protocol.

➤ Performs automatic redistribution between IGRP and EIGRP when using

the same AS.

➤ Requires less CPU resources compared to IGRP. By default, EIGRP allows

up to 50 percent of the bandwidth.

➤ Carries subnet information in updates, which means support for VLSM.

➤ Supports authentication (in IOS release versions 11.3+).

Table 6.2

Summary of IGRP commands

(continued).

Command

Description

passive-interface <interface>

Stops IGRP updates from being sent out only

router igrp <AS>

Runs IGRP with an autonomous number required

timers basic <update>

Modifies IGRP timers

<invalid><holdown> <flush>

variance <value>

Specifies load balancing over unequal cost paths

default-metric bandwidth delay

Specifies the default metrics when redistributing

reliability loading mtu

debug ip igrp events

Provides routing messages that are sent
and received

debug ip igrp transaction

Displays the transactions that are being handled

○

Routing and Switching Algorithms

To ensure that routing information is valid, EIGRP uses several components:

➤ Protocol dependant modules—Provides support for other routable protocols,

such as IPX and AppleTalk.

➤ Reliable transport protocol—Ensures the delivery of EIGRP packets, which in

turn leads to reliable routing tables. Packets are sent to the reserved class D
address 224.0.0.10.

➤ Neighbor discovery/recovery—Enables EIGRP to discover new neighbors on

any network segment. EIGRP will discover when neighbors are no longer
reachable and tear down any routes that originated from unreachable neigh-
bors. When a neighbor has been discovered, the two routers send each other
their reachable networks. EIGRP will use the least-cost path to a network.
The neighboring router is termed the feasible successor. Any other router
that provides the same route information, which also provides a loop free
path, is called a feasible successor. EIGRP will determine which path has the
lowest metric and that router will be chosen as the successor. If this router
goes down for some reason, EIGRP will attempt to calculate a new path.

Configuration of EIGRP

Once again, let’s refer the routers shown in Figure 6.1 for illustrative purposes. In
this section, the routers will be configured for EIGRP. To do so, you must change
the routing process to EIGRP on all four routers using the following command:

router eigrp 1

network 10.0.0.0

Listing 6.10 shows the IP routing table for router R1 after EIGRP is configured.

Listing 6.10

The show ip route command after enabling EIGRP.

R1#sh ip route

10.0.0.0/24 is subnetted, 9 subnets

D 10.1.9.0 [90/40537600] via 10.1.4.1, 00:00:55, Serial3

D 10.1.8.0 [90/41024000] via 10.1.4.1, 00:00:52, Serial3

[90/41024000] via 10.1.2.2, 00:00:52, Serial2

D 10.1.3.0 [90/40537600] via 10.1.2.2, 00:00:52, Serial2

C 10.1.2.0 is directly connected, Serial2

C 10.1.1.0 is directly connected, Ethernet0

C 10.1.7.0 is directly connected, Serial1

D 10.1.6.0 [90/40537600] via 10.1.7.2, 00:00:52, Serial1

D 10.1.5.0 [90/41024000] via 10.1.7.2, 00:00:55, Serial1

[90/41024000] via 10.1.2.2, 00:00:55, Serial2

C 10.1.4.0 is directly connected, Serial3

○

Chapter 6

Notice in Listing 6.10 that the networks are reachable via D (EIGRP) and the
administrative distance is 90. The metric is 256 times what IGRP will calculate.
When IGRP was running, the cost calculated by router R1 to the network 10.1.6.0
was 158350. With EIGRP started, the metric is 40537600 or 256*158350.

To display the characteristics of EIGRP, issue the show ip protocol IOS command.
Listing 6.11 provides the output of this command when issued on router R1.

Listing 6.11

The show ip protocols command after enabling EIGRP.

R1#sh ip protocols

Routing Protocol is "eigrp 1"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 1

Automatic network summarization is in effect

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.2.2 90 00:05:52

10.1.7.2 90 00:05:52

10.1.4.1 90 00:05:52

Distance: internal 90 external 170

As you can see in Listing 6.11, the AS is 1, and there is no update interval be-
cause only changes are sent by EIGRP. Also, the administrative distance (AD)
for EIGRP is 90 for internal routes, and 170 for external routes. External routes
are networks injected into an EIGRP domain by another routing protocols, such
as RIP.

When using EIGRP, you might want to display information about neighboring
routers. To do so, you can use the show ip eigrp neighbor command. This com-
mand displays current routers in the same AS also running EIGRP—the listed
routers share information between each other or form adjacencies in order to
facilitate this exchange.

Listing 6.12 provides the status of adjacencies on router R1 we, which should
include the other three routers (R2, R3, and R4).

○

Routing and Switching Algorithms

Listing 6.12

The show ip eigrp neighbors command.

R1#sh ip eigrp neighbors

IP-EIGRP neighbors for process 1

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms)

2 10.1.2.2 Se2 13 00:09:21 36 2280 0 8

1 10.1.7.2 Se1 13 00:09:31 40 2280 0 6

0 10.1.4.1 Se3 10 00:09:52 639 3834 0 10

By default, EIGRP automatically summarizes IP classes. This means that if you
are using a Class A address to cross a network boundary then EIGRP will install
this network with a Class A mask of 255.0.0.0. You can disable automatic sum-
marization by using the no auto-summary IOS command. You can also manu-
ally summarize a network as a classless route by applying the ip summary-address
eigrp <AS> <network> <mask> interface command. Table 6.3 provides a sum-
mary of common EIGRP IOS commands.

Now, let’s move on to a more advanced type of routing protocol, namely link-
state protocols. We will begin by discussing Intermediate System to Intermediate
System (ISIS) protocol and then we’ll take a look at Open Shortest Path First
(OSPF).

Table 6.3

Summary of EIGRP commands.

Command

Description

auto-summary

Enables auto summarization. This is the default
action.

Maximum-paths

Sets the maximum paths. The default is 4.

metric maximum-hops <hops>

Specifies the maximum hops EIGRP will use. The
default is 100, and the range is from 1 through 255.

ip summary-address eigrp

Enables summarization.

autonomous-system-number
address mask

network <network number>

Runs EIGRP on the associated interface.

passive-interface <interface>

Stops EIGRP updates and hello packets from being
sent out.

router eigrp <AS>

Runs EIGRP with autonomous number required.

Show ip eigrp neighbors

Displays neighbors in the same AS.

variance <value>

Specifies load balancing over unequal cost paths.

Bandwidth

Specifies the parameter used for metric calculation.

ip bandwidth-percent eigrp

Configures the maximum allowable bandwidth to be
used by EIGRP packets. The default is 50 percent of
the bandwidth.

○

Chapter 6

Intermediate System to Intermediate
System (ISIS)

Intermediate System to Intermediate System (ISIS) is a link-state protocol. Cisco’s
implementation of ISIS will populate a routing table with remote IP networks.
ISIS will form a link-state database and will only send out updates when a net-
work event has occurred.

Note: ISIS and OSPF are link state protocols used by Cisco routers. Link-state refers
to the state of an interface, including the status of the interface, IP address, subnet
mask, and network type. All these bits of information describe the state of the
interface, or the link state.

The main characteristics of ISIS include the following:

➤ Sends out hello packets that discover new neighbors. A hello packet is a multi-

cast packet that is used by routers for discovering neighboring devices, such
as routers.

➤ Maintains a link-state database.

➤ Summarizes networks to reduce the size of routing tables.

➤ Serves as a classless protocol (which means ISIS supports VLSM).

➤ Authenticates IP routes.

A Cisco router running ISIS can be an intermediate system (IS) or an end system
(ES). An IS node is simple a routing node in an OSI network. An ES node is gener-
ally an end-user device on a network. Figure 6.2 shows a typical ISIS environment.

As you can see in Figure 6.2, the communication between end systems and an
intermediate system is identified as an ES-IS connection. Communication be-
tween IS end systems is referred to as an ISIS connection. Figure 6.2 also defines
areas in the ISIS environment that reduce the routing table’s size and memory
requirements—namely, router R1 in area 1 and R2 in area 2. Routers that have
ES-IS and ISIS connections will maintain a different database for each connec-
tion. Further, routers in the same area will maintain a Level 1 (L1) database,
while routers in different areas will need to maintain Level 1 and Level 2 (L1/
L2) databases. These are the link-state database used by ISIS.

Configuration of ISIS

To enable ISIS on a Cisco router, you must perform the following configura-
tion tasks:

➤ Enable ISIS with the router isis command.

○

Routing and Switching Algorithms

➤ Configure ISIS interface parameters.

➤ Configure area parameters.

➤ Configure any miscellaneous parameters.

The three methods (referred to as network entities) used to define an ISIS area
and their field formats are:

➤ Simple—Area System ID SEL

➤ OSI—Domain Area System ID SEL

➤ GOSIP—AFI ICD DFI AAI Reserved RDI Area System ID SEL

The meaning of each network entity field is defined as follows:

➤ AFI—Authority and format identifier (47, for Cisco routers)

➤ ICD—International code designator

➤ DFI—Domain-specific part

➤ AAI—Administrative authority identifier

➤ RDI—Routing domain identifier (an autonomous system number)

➤ SEL—Selector byte used to ensure uniqueness in the address (NSAP is Net-

work Service Access.)

Router

ES-IS

Token

Ring

ES-IS

ISIS

Area 1

Area 2

Figure 6.2

ISIS terminolgy.

○

Chapter 6

➤ Area—This value is used by level 2 routers

➤ System ID—Used by level 1 routers typically an interface MAC-address

Note: You are not be expected to remember these formats. They are included here for
your reference only.

Let’s configure the routers in Figure 6.2 for ISIS and place each router in the
same domain. We’ll use the simple address format and enable ISIS on all inter-
faces. The area ID on all four routers will be set to 47. Listings 6.13 through 6.16
display the ISIS configuration on routers R1, R2, R3, and R4.

Listing 6.13

ISIS configuration on R1.

hostname R1

clns routing

interface Ethernet0

ip address 10.1.1.1 255.255.255.0

ip router isis

interface Serial1

ip address 10.1.7.1 255.255.255.0

ip router isis

bandwidth 64

clockrate 64000

interface Serial2

ip address 10.1.2.1 255.255.255.0

ip router isis

interface Serial3

ip address 10.1.4.2 255.255.255.0

ip router isis

router isis

net 47.0000.0c8e.774b.00

Listing 6.14

ISIS configuration on R2.

Hostname R2

clns routing

interface Ethernet0

ip address 10.1.3.1 255.255.255.0

ip router isis

bridge-group 1

interface Serial0

ip address 10.1.5.1 255.255.255.0

ip router isis

○

Routing and Switching Algorithms

bandwidth 64

no fair-queue

clockrate 64000

interface Serial1

ip address 10.1.8.2 255.255.255.0

ip router isis

bandwidth 64

clockrate 64000

bridge-group 1

interface Serial3

ip address 10.1.2.2 255.255.255.0

ip router isis

router isis

net 47.0000.0c75.cf24.00

Listing 6.15

ISIS configuration on R3.

Hostname R3

clns routing

interface Ethernet0

ip address 10.1.6.1 255.255.255.0

ip router isis

interface Serial0

ip address 10.1.7.2 255.255.255.0

ip router isis

interface Serial1

ip address 10.1.5.2 255.255.255.0

ip router isis

router isis

net 47.aa00.0400.0120.00

Listing 6.16

ISIS configuration on R4.

Hostname R4

clns routing

interface Ethernet0

ip address 10.1.9.1 255.255.255.0

ip router isis

interface Serial0

ip address 10.1.8.1 255.255.255.0

ip router isis

○

Chapter 6

interface Serial1

ip address 10.1.7.1 255.255.255.0

bandwidth 64

interface Serial2

ip address 10.1.4.1 255.255.255.0

ip router isis

router isis

net 47.0000.0c75.d97e.00

As you can see in the preceding four listings, each router has a defined network
entry and the ISIS enabled interfaces that will listen and send ISIS updates.

Listing 6.17 displays the IP routing table from R1.

Listing 6.17

The sh ip route command after configuring ISIS on R1.

R1#sh ip route

10.0.0.0/24 is subnetted, 9 subnets

i L1 10.1.9.0 [115/20] via 10.1.4.1, Serial3

i L1 10.1.8.0 [115/20] via 10.1.4.1, Serial3

[115/20] via 10.1.2.2, Serial2

i L1 10.1.3.0 [115/20] via 10.1.2.2, Serial2

C 10.1.2.0 is directly connected, Serial2

C 10.1.1.0 is directly connected, Ethernet0

C 10.1.7.0 is directly connected, Serial1

i L1 10.1.6.0 [115/20] via 10.1.7.2, Serial1

i L1 10.1.5.0 [115/20] via 10.1.7.2, Serial1

[115/20] via 10.1.2.2, Serial2

C 10.1.4.0 is directly connected, Serial3

As you can see in Listing 6.17, the remote networks are reachable through ISIS.
The administrative distance for ISIS is 115, and the metric is 20. The left-hand
side of the displays shown with lower case letter i, which indicates ISIS is the
routing protocol used to reach the remote network. You can display the protocol
characteristics on any ISIS router by using the show ip protocols IOS command,
as shown in Listing 6.18.

Listing 6.18

The show ip protocols command after enabling ISIS.

Routing Protocol is "isis"

Sending updates every 0 seconds

Invalid after 0 seconds, hold down 0, flushed after 0

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: isis

○

Routing and Switching Algorithms

Address Summarization:

None

Routing for Networks:

Ethernet0

Serial1

Serial2

Serial3

Routing Information Sources:

Gateway Distance Last Update

10.1.2.2 115 00:05:16

10.1.7.2 115 00:05:17

10.1.4.1 115 00:05:17

Distance: (default is 115)

Notice in Listing 6.18 that updates are not sent unless a change occurs, like any
link-state protocol. To configure a default route, you use the default-information
originate command; otherwise, the default router will not be advertised. Listing
6.19 outlines the configuration on router R4 required to advertise a default route.

Listing 6.19

Default route configuration on router R4.

hostname R4

router isis

net 47.0000.0c75.d97e.00

default-information originate

ip route 0.0.0.0 0.0.0.0 null0

!Injects a Default route

The routing table for router R1, shown in Listing 6.20, shows that a default route
has been installed into the routing table.

Listing 6.20

The show ip route command showing a default route on router R1.

R1#sh ip route

.....

i L1 10.1.5.0 [115/20] via 10.1.7.2, Serial1

[115/20] via 10.1.2.2, Serial2

C 10.1.4.0 is directly connected, Serial3

i*L2 0.0.0.0/0 [115/10] via 10.1.4.1, Serial3

As you can see in Listing 6.20, the default router for R1 is via Serial 3 or router R4.

There are many configuration and display options for ISIS. Table 6.4 provides a
summary of the major commands available on a Cisco router when running ISIS.

Another very popular link-state protocol is Open Shortest Path First (OSPF).

○

Chapter 6

Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) is a link-state protocol used by internal net-
works. This means that OSPF distributes information between routers belong-
ing to the same autonomous system. OSPF runs over IP enabled networks and
OSPF has been defined in many RFCs over the years.

OSPF was originally developed in RFC 1131, and its most recent specifications
are in RFC 2328. OSPF was designed to handle large IP networks and manage
them into smaller networks called areas. Areas are used to reduce memory and
CPU requirements. OSPF is much more process intensive than RIP, IGRP, and
EIGRP.

The popularity of OSPF ensures that it will remain as a primary routing protocol
for some years to come. Figure 6.3 illustrates a brief history of OSPF, beginning
with its inception in 1989.

The main features of OSPF include the following:

➤ Supports VLSM and classless behavior.

➤ Uses a metric based on a cost value. The formula used is 10

/Bandwidth in

BPS.

➤ Supports equal cost load balancing up to six paths.

➤ Uses hello packets to discover and maintain links to other routers, which

reduces bandwidth requirements.

➤ Supports authentication.

Table 6.4

Summary of ISIS commands.

Command

Description

router isis

Enables ISIS.

net entity

Configures the network for the router.

ip router isis

Enables ISIS on a local interface.

isis metric

Sets the metric used by ISIS for Level 1 or 2 interfaces.

isis hello-interval

Sets the hello interval in seconds.

isis password

Sets the password used in authentication.

default-information originate

Advertises a default route.

summary-address

Summarizes networks.

show isis database

Displays the ISIS link-state database.

show isis spf-log

Displays the number of times the SPF algorithm has
been run due to network changes.

○

Routing and Switching Algorithms

➤ Provides fast convergence.

➤ Provides network summarization.

➤ Supports dial-on-demand links.

Before you look at an example of enabling OSPF, you should review some of the
key concepts and terminology used in OSPF networks.

OSPF Key Concepts and Terminology

In this section, we will discuss concepts and terminology that are used in OSPF
network. Namely, this section addresses the following topics:

➤ Adjacency

➤ Router ID

➤ Shortest path first (SPF) Algorithm

➤ Area border routers (ABRs)

➤ Virtual links

➤ Authentication

➤ OSPF over demand circuits

➤ OSPF network types and broadcast media

➤ Designated and backup designated routers

OSPF,

version 1

RFC 1131

Interaction

with BGP

OSPF over

dial-up

support added

OSPF,

version 2

MIB added

OSPF,

version 2

updated in

RFC 1583

OSPF,

version 2

updated in

RFC 2178

(current)

OSPF

version 2

updated in

RFC 2328

1989

1991

1992

1994

1995

1997

1998

Figure 6.3

The evolution of OSPF.

○

Chapter 6

OSPF Adjacency

What is an adjacency? Basically, adjacency occurs when two routers running OSPF,
for instance, have discovered each other and exchanged routing information. This
communication is bidirectional (both ways). To view the adjacency on a Cisco
router, you use the show ip ospf neighbor command. Listing 6.21 provides an
example of adjacency taken from a Cisco router with one neighbor.

Listing 6.21

The show ip ospf neighbor command showing adjacency.

R1#sh ip ospf nei

Neighbor ID Pri State Dead Time Address Interface

10.1.9.1 1 FULL/ - 00:00:39 10.1.4.1 Serial3

The sample display shows the following fields:

➤ Neighbor ID—The remote router’s ID, which is unique in the domain.

➤ Pri—IP OSPF priority of neighbor.

➤ State—OSPF state (other states are described in the following text).

➤ Dead time—Expected time before Cisco IOS software will declare that the

neighbor is dead.

➤ Address—IP address of the local interface.

➤ Interface—The interface on which the neighbor exists (Serial3 in the preced-

ing example).

There are a number of OSPF states between routers. The OSPF state indicates
the relationship between two OSPF-speaking routers. The available states include:

➤ Down—No information has been received from any device.

➤ Attempt—No recent information has been received from the neighbor.

➤ Init—The interface has detected a hello packet coming from a neighbor, but

bidirectional communication has not yet been established.

➤ Two-way—Bidirectional communication with a neighbor exists.

➤ Exstart—Routers are trying to establish the initial sequence number that is

going to be used in the information exchange packets. The sequence number
ensures that routers always get the most recent information. One router will
become the primary router and the other will become secondary. The primary
router will poll the secondary for information.

➤ Exchange—Routers exchange entire link-state databases.

➤ Loading—Routers are finalizing the information exchange.

➤ Full—Adjacency is completed. At this stage, the routers have the same database.

○

Routing and Switching Algorithms

OSPF Router ID

A router ID is used by OSPF to distinguish the routing process from other rout-
ers. A router ID must be unique across an OSPF network. This ID is the highest
IP address on the router, or if you use a loopback interface, it is the highest address
assigned by any loopback. A loopback interface is a software (logical) interface
that will never fail because the loopback interface has no physical cable connec-
tion subject to failure like an Ethernet interface for example. If a network uses a
serial line IP address and it continually fails, then OSPF would need to restart
every time there is a failure. Therefore, it is highly recommended to use a loopback
address when using OSPF to maintain network stability. Listing 6.22 displays
the output when you view the router ID on a Cisco router, using the show ip ospf
database command.

Listing 6.22

The show ip ospf database command showing the router ID.

R1>sh ip ospf database

OSPF Router with ID (10.1.7.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.1.7.1 10.1.7.1 624 0x80000007 0x5F7B 6

10.1.9.1 10.1.9.1 621 0x80000004 0x156E 4

The router ID for this router is the address 10.1.7.1. This is the highest IP ad-
dress configured on the router, which happens to be Serial 1. If you configured a
loopback address using the address 9.1.1.1, the router ID would change to 9.1.1.1,
as shown in Listing 6.23.

Listing 6.23

The show ip ospf database command.

R1#sh ip ospf data

OSPF Router with ID (9.1.1.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

9.1.1.1 9.1.1.1 13 0x80000003 0xD718

10.1.7.1 10.1.7.1 35 0x80000008 0x4B91

10.1.9.1 10.1.9.1 17 0x80000005 0xD5B3

Notice the router ID is now 9.1.1.1, even though this address is the lower than the
10.7.1.1 address. Loopbacks are always chosen first because of their reliability.

Shortest Path Tree (SPT)

Another OSPF concept you need to be aware of is the shortest path tree. Each
OSPF router will place itself at the root of the tree and determine the least cost
path to all networks. Figure 6.4 provides an example calculation of OSPF cost.

○

Chapter 6

In Figure 6.4, router R1 has two connections to routers R2 and R3. The cost on
these links are 10 and 40, respectively. Router R1 will form a tree with itself as the
root and calculate the cost to the remote networks as follows:

10.1.2.0/24 as 10+20=30 and 10.1.3.0/24 as 40+10=50

Cisco’s routers running OSPF will keep track of up to six equal cost paths to the
same destination. After the router builds the shortest path tree, it starts to build
the routing table accordingly. Any directly connected networks will be reached
via a metric (cost) of 0.

Area Border Routers (ABRs) and Virtual Links

As previously mentioned, OSPF uses areas to limit memory and CPU require-
ments. A that router can be in one area is termed an internal router, a router in
multiple areas is termed an area border router. A router that is also part of an-
other routing domain is termed an autonomous system area border router (ASBR).
Finally, there is another area in OSPF that is usually present in all networks—the
backbone area, or the area designated as Area 0.

Areas are used to stop the explosion of link-state updates. Flooding and calcula-
tion of the Dijkstra algorithm on a router is limited to changes within an area. All
routers within an area have the exact link-state database. Routers can be more
than one type of OSPF route, as illustrated in Figure 6.5.

10.1.1.0/24

Cost = 20

10.1.2.0/24

Cost = 10

10.1.3.0/24

Cost = 10

Cost = 40

Cost = 30

Cost = 50

Router R1 calculates
least cost path with
itself as the root of
a tree

10.1.4.0/24

10.1.5.0/24

Figure 6.4

Shortest Path Tree example.

○

Routing and Switching Algorithms

In Figure 6.5, you can see three areas—Area 0 (the backbone area), Area 1, and
Area 2. Routers R1 through R4 are in Area 0, routers R5 through R7 are in Area 1,
and routers R8 through R10 are in Area 2. All routers within a single area only
are called internal routers. In Area 0, the backbone routers (also called internal
routers) include R1 and R3, because those two routers only connect to a single
area. Routers connected to multiple areas are called area border routers (ARBs). In
Figure 6.5, routers R2, R4, R5, R8, and R9 are area border routers. Routers con-
necting to external routing domains are autonomous system boundary routers
(ASBRs). In Figure 6.5, the router named R10 can be classified as an ASBR.

Routing information that is exchanged between the various OSPF router types
are termed as follows:

➤ Router links—Sent by all routers. Describes the state and cost on the router’s

own links.

➤ Network links—Generated by routers sharing the same media, such as Ethernet

or Token Ring. Sent by internal routers.

➤ Summary links—Sent by ABRs only. Describes networks to other areas but

under the same autonomous system

Network

Lines

Router

Links

Area 0 or Backbone

Router

Links

Area 1

BGP

Domain

or the

Internet

ISP

Network

Lines

R10

Router

Links

Area 2

External

Links

LSA type

Router links

Network links

Summary links

3 or 4

External links

NSSA

Types of Link

State Advertisements

Figure 6.5

OSPF router types and link state packets types.

○

Chapter 6

➤ External links—Sent by ASBRs only. Describes networks to external routers

not in the same autonomous system. External routes can be external type 1 or
external type 2. The difference between the two types lies in the way the cost
(metric) of the route is calculated. The cost of a type 1 route is determined by
adding the external cost and the internal cost used to reach the route. The
cost of a type 2 route always equals the external cost, because the internal cost
is irrelevant. OSPF type 1 routes are always preferred over type 2 routes for
the same destination, because the cost will always be lower.

Networks learned from routers in the same area are called

intra-area

routes, networks learned via other areas are termed inter-area routes.
Cisco routers always choose an intra-area route before an inter-area,
followed by external networks.

There are two more area types that can be defined in OSPF domains—stubby
and not so stubby areas (NSSAs). Similar to the previously discussed areas, these
area types are used to reduce the memory and CPU requirements in OSPF net-
works. A stub area is where you have a single entry and exit port. This setup helps
to reduce advertisements in the area and the backbone. Not so stubby areas (NSSA)
allow external routes to be advertised with but also have the same characteristics
of a stub area. As mentioned earlier, external routes are networks that have been
learned from another routing protocol that have been redistributed into OSPF.

All routers in a single area must have at least one router connected to the back-
bone area for OSPF routes to be injected into the routing table. If a situation
exists like the one in Figure 6.6, then you can use a virtual link. A virtual link can
overcome the limitation of having to connect to the backbone by creating a con-
nection between areas not directly connected to the OSPF backbone area 0.0.0.0.
This is not the best solution, though, and is typically only used during a migra-
tion phase. The network shown in Figure 6.6 requires a virtual link.

In Figure 6.6, the routers in area 2 will not be able to see the backbone area and
vice versa, because, in OSPF, all areas must be connected to the backbone. In this
type of situation, you could use one of two options to overcome the limitation of
not connecting remote areas to the OSPF backbone area or to an ABR router.
You could install a link between area 2 and the backbone, or you could configure
a virtual link between area 2 and the backbone.

Virtual links are used for two purposes:

➤ Linking an area that does not have a physical connection to the backbone.

➤ Patching the backbone in case discontinuity of area 0 occurs.

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Routing and Switching Algorithms

OSPF Authentication

OSPF has the capability of authenticating all packets that are exchanged be-
tween adjacent neighbors. Authentication ensures that only trusted routers will
send and receive IP network information. Cisco routers support plain text and
MD5 authentication.

To enable authentication, follow these two simple steps:

1. Enable the authentication on the interface.

2. Enable authentication in the ospf area.

We will cover the IOS command set a little later in this chapter when we look at
the code used to configure OSPF.

OSPF over Demand Circuits

OSPF routers discover their neighbors using the hello protocol, similar to other
link-state protocols. Hello protocols send and receive hello packets over a set in-
terval. For example, on a Basic Rate Interface (BRI), the default hello interval is
10 seconds.

Area 0 (Backbone)

Area 1

R10

Area 2

Area 0 cannot see Area 2 and vice versa.
Routers in Area 1 can see both domains.

Figure 6.6

Virtual links.

○

Chapter 6

If a hello packet is not received by a value of four times the hello interval (this is
called the dead interval), then the adjacency (two routers that have exchanged
their full database) will be torn down. In this way, hello packets can cause a dial-
up link to remain active, even if no data is being sent. OSPF over a demand
circuit provides a solution to this problem. OSPF over demand circuits stops
hello packets after the two routers have exchanged their database. Then, the link
is only brought up if data is transferred. This saves the on WAN costs. The en-
able OSPF on demand issue the ip ospf demand-circuit IOS command.

OSPF Network Types and Broadcast Media

OSPF supports several network types, including Ethernet and Frame Relay. This
flexibility enables OSPF to run on any network, including Ethernet, Token Ring,
Frame Relay, and X25. OSPF uses multicasts (multicasts are frames sent to a
unique address that are recognized by multicast clients as destined for them) to
discover and maintain neighbors. If a multicast cannot access a medium, an adja-
cency won’t be formed and no IP networks will be injected into the routing tables.

Cisco supports a number of network types, including broadcast, non-broadcast,
and multipoint networks. Broadcast media do not require any additional con-
figuration, because new routers are discovered via the hello protocol. Non-broad-
cast media require manual configuration with the neighbor <ip address> command.
Multipoint interfaces look like a number of point-to-point interfaces with each
device advertising its IP address.

Designated and Backup Designated Routers

Broadcast networks, such as Ethernet, might provide a problem for network de-
signers who do not want every device running OSPF on a local LAN to form an
adjacency (if you had 10 routers on a local Ethernet running OSPF, there would
be 9 adjacencies per router, or 45 connections!). Fortunately, OSPF is designed to
reduce this requirement by having one router serve as the designated router (DR)
and another as the backup designated router (BDR).

With a DR and BDR, the DR establishes an adjacency with all routers, and the
BDR waits until the DR fails to step in and do the same function, such as dis-
seminating information among the local routers to reduce bandwidth require-
ments. The BDR does this almost instantaneously. The function of the DR is to:

➤ Generate network link advertisements on behalf of the network, thereby re-

ducing advertisements.

➤ Disseminate information between the local routers and synchronize updates.

○

Routing and Switching Algorithms

Note: All OSPF enabled routers listen to segments sent to the Class D reserved
address of 224.0.0.5.

When a local router that is not the DR/BDR sends a new update via a link-state
packet (LSA), the router will only send the update to the DR/BDR. The DR then
notifies all other routers. This process reduces memory and CPU requirements. The
DR is selected based on the router with the highest priority. The second-highest
priority router becomes the BDR. In the event of a tie, the router with the highest IP
address becomes the Designated Router (DR). The priority can be set per interface
with the ip ospf priority <0-255> IOS command. If the priority value is set to 0, the
local router will not be involved in the election of the DR or BDR. This assumes all
rotuers are enabled at the same time. Once a router is the DR or BDR changing the
priority will only effect new adjancies not existing ospf adjancies.

You can make a router avoid becoming a DR or BDR by setting the
router’s priority to 0 with the ip ospf priority 0 IOS command.

Configurating OSPF

Now that you have OSPF’s key concepts and terminology mastered, you are ready
to configure OSPF. Configuring OSFP on a Cisco routers entails two main steps:

1. Enable OSPF with the router ospf <process id> command.

2. Identify the interfaces that will be OSPF with the network address wildcard

mask area command. The wildcard mask indicates which bits are pertinent
(0 bits) and which bits aren’t (1 bits). For example, the wildcard mask
0.0.255.255 indicates that the router must match the first two bytes, but the
last two bytes are irrelevant and are included in routing protocol updates.

Let’s configure the network shown in Figure 6.7 for OSPF.

In this configuration, we’ll assign the WAN links to be in the backbone and the
Ethernet networks in area 1 for router R1, area 2 for router R2, and area 3 for
router R3. We’ll also use MD5 authentication in the backbone and assign a pro-
cess ID (local significance only) as 1 for R1, 2 for R2, and 3 for R3. This OSPF
process number is not the autonomous system number. Listings 6.24 through
6.26 show the configurations for routers R1, R2, and R3 with the described setup.

○

Chapter 6

Listing 6.24

Configuration for router R1 with OSPF.

hostname R1

interface Ethernet0

ip address 10.1.6.1 255.255.255.0

interface Serial0

ip address 10.1.7.2 255.255.255.0

ip ospf message-digest-key 1 md5 ccie

interface Serial1

ip address 10.1.5.2 255.255.255.0

ip ospf message-digest-key 1 md5 ccie

10.1.6.0/24

Area 1

Area 2

Area 3

10.1.1.0/24

10.1.3.0/24

10.1.2.0/24

Area

0.0.0.0

10.1.7.0/24

10.1.5.0/24

OSPF backbone
with authentication,
password is ccie

Figure 6.7

Advanced OSPF configuration.

○

Routing and Switching Algorithms

router ospf 1

network 10.1.6.0 0.0.0.255 area 1

network 10.1.7.0 0.0.0.255 area 0

network 10.1.5.0 0.0.0.255 area 0

area 0 authentication message-digest

Listing 6.25

Configurations for router R2 with OSPF.

hostname R2

interface Ethernet0

ip address 10.1.1.1 255.255.255.0

interface Serial1

ip address 10.1.7.1 255.255.255.0

ip ospf message-digest-key 1 md5 ccie

router ospf 2

network 10.1.1.0 0.0.0.255 area 2

network 10.1.7.0 0.0.0.255 area 0

network 10.1.2.0 0.0.0.255 area 0

area 0 authentication message-digest

Listing 6.26

Configurations for router R3 with OSPF.

hostname R3

interface Ethernet0

ip address 10.1.3.1 255.255.255.0

interface Serial0

ip address 10.1.5.1 255.255.255.0

ip ospf message-digest-key 1 md5 ccie

interface Serial1

ip address 10.1.2.2 255.255.255.0

ip ospf message-digest-key 1 md5 ccie

router ospf 3

network 10.1.3.0 0.0.0.255 area 3

network 10.1.5.0 0.0.0.255 area 0

network 10.1.2.0 0.0.0.255 area 0

area 0 authentication message-digest

Listing 6.27 provides the routing table from the viewpoint of router R1.

Listing 6.27

The show ip route command for router R1 with OSPF configured.

R1#sh ip route

10.0.0.0/24 is subnetted, 6 subnets

O IA 10.1.3.0 [110/1572] via 10.1.5.1, 00:09:31, Serial1

O 10.1.2.0 [110/3124] via 10.1.5.1, 00:09:32, Serial1

[110/3124] via 10.1.7.1, 00:09:32, Serial0

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Chapter 6

O IA 10.1.1.0 [110/1572] via 10.1.7.1, 00:09:32, Serial0

C 10.1.7.0 is directly connected, Serial0

C 10.1.6.0 is directly connected, Ethernet0

C 10.1.5.0 is directly connected, Serial1

As you can see in Listing 6.27, the remote networks in areas 2 and 3 are denoted
by O IA on the left-hand side, which means they are inter-area routers. O de-
notes IP networks in the same area for OSPF. These networks are intra-area
routes. The administrative distance for OSPF is 110, and the cost is paired to-
gether in the square brackets. The cost to reach network 10.1.3.0 is 1572.

Notice the process ID is different in all three routers. Listing 6.28 displays some
of the vital information about this OSPF network.

Listing 6.28

The show ip ospf int command after configuring OSPF.

R1#sh ip ospf int s 0

Serial0 is up, line protocol is up

Internet Address 10.1.7.2/24, Area 0

Process ID 1, Router ID 10.1.7.2,...

…Network Type POINT_TO_POINT, Cost: 1562

Transmit Delay is 1 sec, State POINT_TO_POINT,

Timer intervals configured, Hello 10, Dead 40,

Wait 40, Retransmit 5

Hello due in 00:00:03

Neighbor Count is 1, Adjacent neighbor count is 1

Adjacent with neighbor 9.1.1.1

Suppress hello for 0 neighbor(s)

Message digest authentication enabled

Youngest key id is 1

The show ip ospf int command displays the characteristics of the interface in-
cluding the hello interval (10 seconds) and the dead interval (4 x 10 = 40 sec-
onds). You can also see in Listing 6.28 that authentication is used on this link.
The state of the interface is POINT_TO_POINT, and network type is also
POINT_TO_POINT.

As mentioned earlier, OSPF supports a number of network types for different
media types, including:

➤ Point to Point—This network is a link between two routers over a physical

interface, such as a serial line.

➤ Broadcast—This network type is typical in LAN environments.

➤ Non-broadcast—The type is typical in non-broadcast media such as X25 net-

works. Neighbor statements are required here to form an OSPF adjacency.

○

Routing and Switching Algorithms

Listing 6.29 displays the OSPF protocol characteristics using the show ip proto-
cols IOS command.

Listing 6.29

The show ip protocols command after configuring OSPF.

R1# show ip protocols

Routing Protocol is "ospf 1"

Sending updates every 0 seconds

Invalid after 0 seconds, hold down 0, flushed after 0

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: ospf 1

Routing for Networks:

10.1.6.0/24

10.1.7.0/24

10.1.5.0/24

Routing Information Sources:

Gateway Distance Last Update

10.1.8.2 110 00:12:22

10.1.9.1 110 00:36:44

9.1.1.1 110 00:12:23

Distance: (default is 110)

As you can see in Listing 6.29, the OSPF default administrative distance is 110.
To display the neighbors when using OSPF, type the show ip ospf neighbor com-
mand, as shown in Listing 6.30.

Listing 6.30

The sh ip ospf neighbor command after configuring OSPF.

R1#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

9.1.1.1 1 FULL/ - 00:00:35 10.1.7.1 Serial0

10.1.8.2 1 FULL/ - 00:00:33 10.1.5.1 Serial1

Listing 6.30 indicates that router R1 it has full visibility of routers R2 and R3. To
set a default route in OSPF, issue the default-information originate command.
Table 6.5 provides a summary of useful OSPF commands.

Table 6.5

Summary of OSPF commands.

Command

Description

network address wildcard

Defines which interfaces will be advertised.

mask area area-id

neighbor ip address

Configures a neighbor statically. Typically used in non-

priority <0-255>

broadcast networks, such as Frame Relay or X.25.

summary-address

Provides external summarization to another

address mask

autonomous system.

(continued)

○

Chapter 6

OSPF FAQ

Because OSPF is widely used, this section presents a quick FAQ-like presenta-
tion to assist in rounding out your knowledge about OSPF.

1. What OSPF packets types are used by Cisco routers?

➤ Hello packets—Used to discover neighbors.

➤ Update packets—Sent via LSA.

➤ LSA packets—Used to send link-state acknowledgements.

➤ LSR packets—Used to send link-state requests.

➤ Database packets—Used to describe a link-state database.

2. What is OSPF’s default hello time interval?

The default is 10 seconds. The dead interval is four times the hello interval.

3. What is the formula used to calculate the OSPF cost of any interface?

The formula is 10

/bandwidth. For Ethernet the default cost is 10

/10

=10.

You can override this default calculation by using the ip ospf cost <value>
IOS command.

Table 6.5

Summary of OSPF commands

(continued).

Command

Description

area area-id range mask

Provides a summarization of OSPF networks.

area area-id stub

Defines an area as a stub area.

[no summary]

area area-id virtual-link

Specifies a virtual link configuration.

router-id

show ip ospf virtual-links

Displays virtual links.

area area-id nssa

Creates a not so stubby area.

show ip route

Displays a routing table.

show ip ospf interface

Displays configuration parameters on interface, such
as hello time.

show ip ospf neighbor

Displays adjacencies

show ip ospf

Displays the number of times SPF have run and which
areas the router is configured for.

show ip ospf database

Displays the topological database. The router ID is
listed here.

Debug ip ospf adj

Turns on OSPF adjacency events.

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Routing and Switching Algorithms

4. Why is MD5 authentication preferred over clear text authentication?

MD5 uses encryption to send a password across the medium, whereas clear
text passwords are visible with a network protocol analyzer.

5. Does OSPF run over FDDI, Token Ring, and ATM LANE,?

Yes. OSPF can run over FDDI.,Token Ring, and ATM LANE.

6. Can OSPF support other protocols, such as AppleTalk and IPX?

No. OSPF is used for routing IP only.

Border Gateway Protocol (BGP)

BGP is another IP routing protocol that enables you to route IP. BGP is defined
in RFCs 1771 and 1267. BGP has evolved from version 1 to version 4. The
earlier versions are now obsolete, and the standard version is 4, or BGP4. BGP
running between autonomous systems is referred to as external BGP, or EBGP,
while running BGP inside an autonomous system it is referred to as internal
BGP, or IBGP.

BGP is the only routing protocol that runs over TCP using port 179. A TCP
connection is manually configured between routers and is referred to as a peer
connection, or the BGP neighbor. After a TCP peer connection is established, the
BGP routers exchange their full routing tables. Keep-alives are then sent to en-
sure that the session stays active, and only topology changes are sent in the future.
BGP uses a separate table called a BGP table to maintain routing links. Only
when an entry is in the BGP table along with other conditions discussed later in
this chapter will a network be inserted into the IP routing table. BGP also sup-
ports CIDR, which allows the routing table to be reduced by using summariza-
tion. In summary, BGP:

➤ Serves as a path-vector protocol.

➤ Runs over a TCP connection using port 179.

➤ Routes between and inside autonomous systems. The AS number must be

unique in the routing domain to stop routing loops. The InterNIC that handles
IP addresses also handles AS for the Internet.

➤ Supports CIDR.

➤ Uses a complex routing decision algorithm based on the 10 conditions, which

will be discussed later in this section on BGP.

Let’s now cover the BGP attributes that are used to select the preferred path to a
remote network.

○

Chapter 6

BGP Attributes

BGP uses certain attributes to define how a network can be reached. These at-
tributes are described as follows:

➤ Next Hop—This is the next hop address that is used to reach a destination

network.

➤ Weight—This value is defined by Cisco, and it is used to determine route

selection. The value ranges from 0 through 65535. The default is 32768. A
higher weight value is a preferred path. The weight is locally significant to a
Cisco router only.

➤ Local preference—This value specifies the preferred path to use to exit the

autonomous system. A higher value is preferred. The default is 100.

➤ Origin—This value defines how a network was originated. The values are:

IGP (learned in the AS) and EGP (learned via the former to BGP, which was
called exterior gateway protocols or incomplete. Incomplete means the net-
work was injected into BGP via another protocols, that is, redistributed. For
example, from RIP to BGP).

➤ AS-PATH—This value indicates what AS the network route has been through.

This value presents a list of AS numbers, such as 100 200 300. In this ex-
ample, the network originated from AS 300 and passed through AS 100 and
AS 200.

➤ Multi Exit Discriminator (MED)—This value supplies a hint to external peers

about a preferred path to a network. A lower MED value is preferred.

➤ Community attribute—This value enables routers to be placed in particular

communities based on shared attributes. Commonly used in large networks.

To more fully explore some of the commonly used attributes, let’s look at a sample
Cisco BGP table. Listing 6.31 shows a BGP table taken from a Cisco 2500 router.

Listing 6.31

The show ip bgp command with BGP configured.

R1# show ip bgp

BGP table version is 2001, local router ID is 131.108.1.199

Status codes: s suppressed, * valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

* i3.0.0.0 192.0.22.1 0 100 32768 1999 1238 ?

* i6.0.0.0 192.0.22.1 0 100 32768 1999 890 1588 ?

*>i7.0.0.0 192.0.16.1 0 100 32768 1800 690 1568 ?

The BGP table shown in Listing 6.31 displays the current BGP version (2001).
As you can see, this version increments every time a new update is received. The

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Routing and Switching Algorithms

local router ID is used by BGP in all updates. The Next Hop indicates the ad-
dress that this network is reachable through. The Metric is the Multi Exit Dis-
criminator. LocPrf specifies the local preference, the Weight is the default of
32768, and the Path specifies which AS networks are required to reach the des-
tination. Each ? after an AS path numbers indicates that the network is injected
into BGP via some other IP routing protocol, such as RIP or IGRP.

By default, a BGP router, such as a Cisco router, does not advertise any externally
learned networks unless they have been leaned via an internal routing protocol.
This stops networks from being advertised and routing black holes. A routing
black hole is where an IP packet is sent to a particular address by a router that
believes the address is the correct destination for the remote network, but no
reply is ever returned. Obviously an administrator does not want data to be lost
so preventing black holes is a real priority in any IP network especially the Internet
or an Internet network, Intranet.

BGP Synchronization

Synchronization occurs when two internal BGP routers have exchanged BGP
routing information and share the same view of a network. A BGP router will
only share information with another BGP router if both routers have learned
about the same networks via an internal routing protocol, such as RIP or OSPF.
BGP Synchronization will stop creating routing black holes, which can occur
when BGP routers do not have a path to a network. By default, synchronization
is enabled on Cisco routers. You can disable synchronization by using the no
synchronization IOS command.

BGP Routing Decision

Cisco’s implementation of BGP goes through 10 routing decisions used to deter-
mine whether to install a BGP table entry into the IP routing table. The 10
decisions, in order, are as described by Cisco IOS documentation:

1. If the next hop is inaccessible, do not consider it.

2. Consider larger BGP administrative weights first.

3. If the routers have the same weight, consider the route with higher local

preference.

4. If the routes have the same local preference, prefer the route from which the

local router originated.

5. If no route was originated, prefer the shorter autonomous system path.

6. If all paths are of the same autonomous system path length, prefer the lowest

origin code (IGP < EGP < INCOMPLETE).

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Chapter 6

7. If origin codes are the same and all the paths are from the same autonomous

system, prefer the path with the lowest Multi Exit Discriminator (MED)
metric. A missing metric is treated as 0.

8. If the MEDs are the same, prefer external paths over internal paths.

9. If IGP synchronization is disabled and only internal paths remain, prefer the

path through the closest neighbor.

10. If all else matches, prefer the route with the lowest IP address value for the

BGP router ID. The router ID is unique.

Configuration of BGP

To enable BGP routing on a Cisco router, use the following IOS command:

router bgp <Autonomous system>

BGP requires you to manually configure a remote neighbor. To do so, you use the
following IOS command:

neigbhor <ip address of remote router> remote-as

<as of remote router>

For illustrative purposes, let’s configure a simple BGP network that contains two
routers—router R1 and router R2—located in AS 1 and 2 respectively. Figure
6.8 shows the topology of the sample BGP network. Listings 6.32 and 6.33 show
the configuration for the two routers.

AS 1

AS 2

131.108.1.1/24

131.108.1.2/24

10.1.1.0/24

10.1.2.0/24

Figure 6.8

Sample BGP configuration.

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Routing and Switching Algorithms

Listing 6.32

Configuring router R1 for BGP.

hostname R1

interface Ethernet0

ip address 10.1.1.1 255.255.255.0

interface Serial0

ip address 131.108.1.1 255.255.255.0

router bgp 1

network 10.1.1.0 mask 255.255.255.0

neighbor 131.108.1.2 remote-as 2

Listing 6.33

Configuring router R2 for BGP.

hostname R2

interface Ethernet0

ip address 10.1.2.1 255.255.255.0

interface Serial0

ip address 131.108.1.2 255.255.255.0

router bgp 2

network 10.1.2.0 mask 255.255.255.0

neighbor 131.108.1.1 remote-as 1

Note: The network command is used to advertise a network that is already in the IP
routing table. If this command is issued with no correspondiong entry in the IP
routing table, BGP will not advertise the network.

After the routers are configured, a TCP session will be establish between routers
R1 and R2. Let’s look at the BGP session to ensure that a TCP peer is estab-
lished. To view the session, you use the show ip bgp neigbour command. Listing
6.34 shows a truncated session. The state of the connection must be established
before a conenction can be successfully set up.

Listing 6.34

The sh ip bgp neigbour command after configuring BGP.

R1#sh ip bgp neigbour

BGP neighbor is 131.108.1.2, remote AS 2, external link

Index 1, Offset 0, Mask 0x2

BGP version 4, remote router ID 131.108.1.2

BGP state = Established, table version = 1, up for 00:00:03

Last read 00:00:03, hold time is 180, keepalive 60 seconds

Minimum time between advertisement runs is 30 seconds

Received 3 messages, 0 notifications, 0 in queue

Sent 3 messages, 0 notifications, 0 in queue

Connections established 1; dropped 0

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Chapter 6

Last reset never

No. of prefix received 0

...

As you can see in Listing 6.34, the BGP peer session from R1 is established. If
any other state keyword displays, then the session has not been set up. Listing
6.35 displays the BGP table on router R1.

Listing 6.35

The show ip bgp command with BGP configured on R1.

R1# show ip bgp

BGP table version is 4, local router ID is 9.1.1.1

Status codes: s suppressed, d damped, h history,

* valid, > best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.1.1.0/24 0.0.0.0 0 32768 i

*> 10.1.2.0/24 131.108.1.2 0 0 2 i

Notice in Listing 6.35 that the remote network 10.1.2.0/24 is reachable through
the next hop address of 131.108.1.2. The right-hand side indicates that this net-
work is learned via i, or via BGP. Do not confuse this with seeing an i on the left-
hand side (which indicates an internal network). The next hop in this network
entry 10.1.2.0/24 will be inserted into the IP routing table, as shown below in
Listing 6.36.

Listing 6.36

The show ip route command with BGP configured.

R1>sh ip route

C 9.1.1.0 is directly connected, Loopback0

10.0.0.0/24 is subnetted, 4 subnets

B 10.1.2.0 [20/0] via 131.108.1.2, 00:02:40

Note the administrative distance of BGP is 20 and the metric, or MED, is 0.

A next hop address of 0.0.0.0 describes a locally connected network
that will be advertised by the BGP speaking router.

BGP Filtering Options

BGP enables you to filter which IP networks are sent and received. Some of the
available filtering methods include prefix filtering, AS_Path filtering, commu-
nity filtering and route-map filtering:

○

Routing and Switching Algorithms

➤ Prefix filtering—Restricts information learned from peer routers based on an

access list.

➤ AS_Path filtering—Restricts traffic based on the AS_Path attribute. For ex-

ample, you can block a network that has been through a particular AS.

➤ Community filtering—Allows you to modify attributes and set conditions.

➤ Route-map filtering—Allows an extensive range of filtering options and are

explored in a little more detail in the following text, due to the advanced
features available.

Route maps can be applied to a number of different scenarios. Route-map filter-
ing is not restricted to BGP—it can be used in redistribution between IP proto-
cols as well. Route-maps can be applied to incoming connections or outgoing
peer connections. You can set parameters or make a match with certain attributes
and set conditions thereafter. Listing 6.37 provides a summary of the full set of
route-map options available.

Listing 6.37

The match ? command.

R1(config-route-map)#match ?

as-path Match BGP AS path list

clns CLNS information

community Match BGP community list

interface Match first hop interface of route

ip IP specific information

length Packet length

metric Match metric of route

route-type Match route-type of route

tag Match tag of route

R1(config-route-map)#set ?

as-path BGP AS-path attribute

automatic-tag Automatically compute TAG value

clns OSI summary address

community BGP community attribute

dampening Set BGP route flap dampening parameters

default Set default information

destination-preference BGP destination preference

interface Output interface

ip IP specific information

level Where to import route

local-preference BGP local preference path attribute

metric Metric value for destination

metric-type Type of metric for destination

origin BGP origin code

tag Tag value for destination

weight BGP weight for routing table

○

Chapter 6

For more on these parameters see the end of this chapter in the “Need to Know
More Section.”

Route-Map Configuration Example

Let’s now modify some route map characteristics using a simple route-map. In
this example, the route-map is named ccie (this name is simply a tag—you can
assign any name you wish) and the route-map will be used to set the weight on
incoming updates to 100, metric to 1, and changes the local preference attribute
to 100.

This example uses the neighbor statement followed by the keyword route-map
and an arbitrary name, or route tag in Cisco terminology. Then, the route-map is
configured globally. For this example, let’s revisit the network illustrated in Fig-
ure 6.8 and place a route-map on router R1. The route-map on R1 will modify all
networks from R2 with a weight of 100, local preference of 100, and a MED of 1.
Listing 6.38 displays the full IOS command and options required for this set up.

Listing 6.38

Configuring a route-map on router R1.

router bgp 1

network 10.1.1.0 mask 255.255.255.0

neighbor 131.108.1.2 remote-as 2

neighbor 131.108.1.2 route-map ccie in

! The router map is applied to incoming updates

! route-map global configuration

route-map ccie permit 10

set metric 1

set local-preference 100

set weight 100

route-map ccie permit 20

In Listing 6.38, the route-map is configured globally and the route tag in this
example is ccie. Also, notice that the route-map is numbered. In this example,
condition 10 will be checked first, followed by 20, and so on. Unlike access lists,
you can delete and install new lines without having to remove an entire route-map.

Looking at the BGP table in Listing 6.39, you can now see that the attributes
have changed to match the attributes specified in the route-map created in List-
ing 6.38.

Listing 6.39

The show ip bgp command with BGP configured after applying
a route-map.

R1#sh ip bgp

BGP table version is 3, local router ID is 9.1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best

, i – internal, Origin codes: i - IGP, e - EGP, ? - incomplete

○

Routing and Switching Algorithms

Network Next Hop Metric LocPrf Weight Path

*> 10.1.1.0/24 0.0.0.0 0 32768 i

*> 10.1.2.0/24 131.108.1.2 1 100 100 2 i

As you can see in Listing 6.39, the incoming updates from R2 for the network
10.1.2.0/24 have been modified so that the local preference is set to 100, metric
is 1, and the weight is set to 100.

Filter-List Configuration Example

You can also apply filters on each TCP peer to allow certain networks into the
BGP table by using the neighbor statement. Let’s now configure a filter-list on
R1 to deny the remote network 10.1.2.0. Listing 6.40 displays router R1’s new
configuration.

Listing 6.40

Applying a filter-list to router R1.

router bgp 1

network 10.1.1.0 mask 255.255.255.0

neighbor 131.108.1.2 remote-as 2

neighbor 131.108.1.2 filter-list 1 in

! Access list that denies remote network 10.1.2.0

access-list 1 deny 10.1.2.0

! Denies specific network

access-list 1 permit any

! Permits any other networks

The BGP table on R1 will not contain the network 10.1.2.0, as illustrated in
Listing 6.41.

Listing 6.41

The show ip bgp command after applying the filter-list to R1.

R1#sh ip bgp

BGP table version is 2, local router ID is 9.1.1.1

Status codes: s suppressed, d damped, h history, * valid,

> best, i - internal

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.1.1.0/24 0.0.0.0 0 32768 i

Let’s now discuss the advanced features of BGP that allow administrators to
scale large BGPO networks, namely BGP route reflectors, clusters, and confed-
erations.

BGP Route Reflectors, Clusters, and Confederations

BGP is a complex routing protocol that requires a fully meshed network. This
means that all internal BGP (IBGP) routers need to have a peer connection to
each other. This does not scale too well if you considered that each router would

○

Chapter 6

require a configuration for each TCP peer. Therefore, the solution to this prob-
lem is to use route reflectors and confederations.

Router Reflector

In large BGP networks, such as the one shown in Figure 6.9, the configuration of
each router within its own AS would required one TCP peer connections. Basi-
cally route reflectors are routers configured to reflect BGP information to other
BGP-speaking routers without the need to configure a TCP peer between each
router. Figure 6.9 displays a typical network scenario where route reflectors and
confederations (which are discussed later in this chapter) are used.

Route reflectors reduce the number of connections required in an internal BGP
network. In the design shown in Figure 6.9, router R6 is the route reflector and
routers R7 through R10 are the route-reflector clients. Together, these routers make

AS 100

AS 200

AS 500

Single Cluster

AS 2000

R10

R11

Figure 6.9

Route reflectors (RR) and confederations.

○

Routing and Switching Algorithms

up a cluster. For example, in AS 500 in Figure 6.9, routers R7, R8, R9, R10, and
R11 are BGP peers to R6 only. Router R6 will convey information received from
AS 100 to all its route reflector clients.

To configure a router-reflector client, you need to add the IOS commands shown
in Listing 6.42.

Listing 6.42

Configuring a router-reflector client for router R6.

R6(config-router)#router bgp 500

R6(config-router)#neighbor 1.1.1.1 remote 1

<CodR6(config-router)#neighbor 1.1.1.1. route-reflector-client

The configuration in Listing 6.42 scales much more elegantly than a fully meshed
network that would require many TCP connections.

Confederations

A confederation is an advanced form of a route reflector, but it is designed to
combat very large internal BGP networks. In Figure 6.9, if you assume that the
whole network is under one common administration, the number of TCP peers
required would be 10 per router. There are 11 routers, so that makes 11*10/2 or
55 connections all together (n*(n-1)/2 were n is the number of routers). Informa-
tion is shared among the autonomous systems but not the internal BGP net-
works unless a TCP peer is established.

In Figure 6.9, three internal autonomous systems make up part of the larger AS,
which is 2000. Each router will need to identify its local AS and the global AS.
Listing 6.43 shows how to configure Router R6 for confederations.

Listing 6.43 Configuring confederations for router R6.

router bgp 500

bgp confederation identifier 2000

bgp confederation peers 100

In Listing 6.43, the first line configures BGP for its local AS, the second line
identifies the global AS, and the last line indicates which of the other routers in
another AS are part of the global AS. The other routers in AS 100 are R1, R2 and R3.

Note: The advanced BGP topics briefly described in this chapter are presented to
introduce you to some of the features BGP can provide. You will need to study BGP
in greater detail and practice as much as you can for the LAB portion of the exam.
You can apply this advice to all the IP routing protocls discussed in this chapter.

Provided below in Table 6.6 is a summary of the most critical IOS commands
used in BGP networks.

○

Chapter 6

There are many ways to change the way BGP selects a path to a destination.
Using BGP enables you to make complex decisions and force BGP to select
routes based on specified criteria. For the examination, you should know about
the following:

➤ Autonomous systems

➤ Confederations

➤ External BGP

➤ Filters

➤ Internal BGP

➤ Path selection

➤ Peer groups

➤ Route maps

➤ Route reflectors

Table 6.6

Common BGP commands.

Command

Description

router bgp <as>

Configures BGP routing.

neighbor <ip address>

Configures remote tcp peer.

remote-as <remote as>

show ip bgp

Display the BGP table.

show ip route

Displays the IP routing table.

network <network number>

Advertises local IP entries into BGP.

mask <network mask>

no synchronization

Disables synchronization in IBGP networks.

aggregate-address <address>

Summarizes BGP networks.

bgp confederation identifier

Specifies an autonomous system.

bgp confederation peers

Identifies a remote peer’s part of a confederation.

autonomous-system

neighbor ip-address

Specifies a route-reflector client.

route-reflector-client

show ip bgp neighbor

Displays BGP peer connections.

clear ip bgp <address>

Clears BGP connections. You can specify an address
or all-peered connection by using the command clear
ip bgp*

○

Routing and Switching Algorithms

BGP FAQ

This section presents a quick FAQ-like presentation to assist in rounding out
your knowledge about BGP.

1. What is BGP dampening?

BGP dampening is a tool that can be used to monitor flapping BGP net-
works and penalize them for each instance. A flapping network is a network
that’s inserted or removed from a routing table because of a consistent net-
work event, such as link down or link up status messages. After a certain
threshold is passed, an entry is removed until it becomes stable once more
and can be used by the router to send user data or routing information. Damp-
ening is enabled with the bgp dampening IOS command.

2. What versions of BGP does Cisco support?

Cisco supports BGP versions 2, 3, and 4. BGP4 is the standard, and the
earlier versions are considered obsolete.

There are numerous ways to move routing information from one domain an-
other. In the next few sections, we’ll briefly cover redistribution and ways to con-
trol how routing information is manipulated.

Redistibuting and Controlling
Routing Information

At this point in the chapter, we have covered all the major protocols that are in
use today. Now, we need to address what happens when one router is running
RIP and another router is running OSPF. What can network administrators do
to enable communication between the network domains? The answer, of course,
is to use redistribution.

Cisco IOS supports redistribution between all IP routing protocols. When you
use redistribution, you have to convert the metric to destination network’s met-
ric. For example, if you were to redistribution from OSPF to RIP, the OSPF cost
value would have little meaning to the RIP domain unless you converted the cost
to a hop count. In other words, you must adhere to the metric used by the proto-
col you are redistributing into. Figure 6.10 shows a simple redistribution between
a RIP domain and an OSPF domain.

In Figure 6.10, router R2 is in both the RIP domain and OSPF domain, so it will
perform the redistribution. The IOS comamnd used to redistribute has the fol-
lowing syntax:

redistribute <protocol>

○

Chapter 6

Listing 6.44 provides the redistribution configuration for router R2.

Listing 6.44

Configuring router R2 for redistribution.

router ospf 1

redistribute rip metric 100

network 131.108.0.0 0.0.255.255 area 0

router rip

redistribute ospf 1 metric 1

network 10.0.0.0

Notice that Listing 6.44 redistributes from the RIP domain to OSPF and vice
versa. The IP routing table on router R2 displays the RIP network 10.1.1.0/8 as
follows:

R1#sh ip route

O E2 10.0.0.0/8 [110/100] via 131.108.1.2, 00:04:06, Serial2

Notice that the metric is set to 100, as specified in the configuration shown in
Listing 6.44.

Note: You must be careful when redistributing between classful and classless domains.
Remember, classful protocols do not send subnet information, so they will always
apply the default mask on any newly discovered networks. If you have a VLSM
network and you redistribute it into a classful domain, then you would need to
summarize the networks using the default mask applied by RIP, for example.

OSPF domain

RIP domain

131.108.1.1/24 131.108.1.2/24

10.1.1.0/24

10.1.2.0/24

Redistribution

performed here

Figure 6.10

Redistribution between RIP and OSPF.

○

Routing and Switching Algorithms

Policy Routing

You can control how a router decides which WAN interface to send an IP packet
to when it has learned dynamically to send it to an interface. To accomplish this
end, you could change the metric that is appropriate for the routing protocols, or
you could use policy routing. Policy routing is where the administrator changes
the default behavior of a router.

To use policy routing, you must first define a route map. The route map must
define what you would like the router to do, and then it must apply the condition
to the appropriate interface with the ip policy route-map <route-map name>
IOS command.

Let’s consider a simple example. Imagine you have a router that has two exit
points and management does not want packets sent to the network to use Serial 0.
You can use an access list to select a range of networks, as illustrated in Listing 6.45.

Listing 6.45

Sample policy route configuration.

interface serial 1

ip policy route-map ccie

access-list 1 permit any

! Global router configuration sets outbound interface

route-map ccie permit 10

match ip address 1

set interface Serial1

To view a router’s policy routing, you can use the debug command debug ip
policy. The output will show you where packets are policy routed and to what
interface.

Note: With policy routing, the IP routing table is unaltered. You must use the debug
ip policy command to view the router’s actions. Any network not included in the
access list will follow normal IP table paths.

Multicast Protocols

In normal data communications, data is sent from one device to another. But, in
some instances, you might want one device to send to many devices. Traditionally,
the sending device would need to send to one end device then another individually.
Multicasting enables the source device to transmit one session to many devices.

Multicasting reduces the bandwidth and time required to transmit from one de-
vice to many devices. When you use multicasting, end devices are configured to
join a multicast group and begin receiving a multicast stream of data. Devices not
configured for multicasting will ignore the multicast frames.

○

Chapter 6

Routers executing a multicast routing protocol, such as Protocol-Independent
Multicast (PIM), maintain forwarding tables to forward multicast datagrams.
Cisco’s implementation of IP multicast routing includes:

➤ Internet Group Management Protocol (IGMP)—Used between hosts on a LAN

and the router(s) on that LAN to track which multicast groups the hosts are
members of.

➤ Protocol-Independent Multicast (PIM)—Used between routers so that they can

track which multicast packets to forward to each other and to their directly
connected LANs.

➤ Distance Vector Multicast Routing Protocol (DVMRP)—Used on the multicast

backbone of the Internet.

So much information to digest! Do not be alarmed by this—there are only 100
questions on the Cisco CCIE exam, and the questions cover a vast amount of
information. Always study your weakest topics first, and then browse what you
think you know really well.

Let’s now test your knowledge with some practice questions. If you get more
than seven incorrect answers, please revisit this chapter and review some of the
resources in this chapter’s “Need to Know More?” section before taking the ex-
amination.

○

Routing and Switching Algorithms

Practice Questions

Question 1

What does RIP stand for?

❍ a. Running Internet Protocol

❍ b. Routed Internet Protocols

❍ c. Routing Informative Protocol

❍ d. Routing Information Protocol

The correct answer is d. RIP stands for Routing Information Protocol. You should
know the full phraseology of protocol acronyms. Answers a, b, and c are incor-
rect, because they do not represent the terms associated with the RIP acronym.

Question 2

Which of the following IP routing protocols are not classful? [Choose the
two best answers]

❑ a. RIP

❑ b. OSPF

❑ c. IGRP

❑ d. ISIS

The correct answers are a and c. RIP and IGRP are classful protocols, while
OSPF and ISIS are classless. The question specifically asks for IP routing proto-
cols that are not classful—make sure you read questions carefully. Answers b and
d are incorrect, because OSPF and ISIS are link-state protocols.

Question 3

What IOS command will enable IP RIP routing?

❍ a. router rip I

❍ b. router rip

❍ c. router-rip

❍ d. enable ip rip

○

Chapter 6

The correct answer is b. To start RIP routing on a Cisco router, the correct syntax
is router rip. Then, you need to define which networks you want to advertise.
Answers a, c, and d are incorrect, because they are invalid IOS commands.

Question 4

What is the correct syntax to use to stop a Cisco router from sending routing
updates out Ethernet0?

❍ a. passive-interface ethernet0

❍ b. passive interface Ethernet0

❍ c. no ip routing

❍ d. enable passive-interface

The correct answer is a. The correct syntax is passive-interface followed by the
interface that will not send updates, such as Ethernet0 or Serial0. Answer b is
incorrect, because the dash is missing. Answer c is incorrect, because the com-
mand will disable IP routing all together. Answer d is incorrect, because the com-
mand is an invalid IOS command.

Question 5

What UDP port number does IP RIP use?

❍ a. 23

❍ b. 520

❍ c. 21

❍ d. 20

❍ e. 17

The correct answer is b. IP RIP runs over UDP port number 520. Answers a, c, d,
and e incorrect, because they do not represent port numbers used by IP RIP.

○

Routing and Switching Algorithms

Question 6

What is the difference between IP RIP I and RIP II? [Choose the two best
answers]

❑ a. RIP II supports 32 hops.

❑ b. RIP II supports VLSM.

❑ c. RIP I supports VLSM.

❑ d. RIP II supports authentication.

The correct answers are b and d. RIP II supports VLSM and authentication,
while RIP I supports neither feature. Answer a is incorrect, because the hop count
is 15 for both RIP I and RIP II. Answer c is incorrect, because RIP I does not
support VLSM.

Question 7

What is the default IGRP

metric

for a 64K line that has a 20,000-microsecond

delay?

❍ a. 6,476

❍ b. 8,476

❍ c. 158,250

❍ d. 156,280

❍ e. More data required

The correct answer is c. The correct formula is IGRP

metric

=10

/64+20000/

10=158,250. Answers a, b, d, and e are incorrect, because the present incorrect values.

Question 8

What IOS command will display the current IP routing protocols in use?

❍ a. show protocols

❍ b. show ip route

❍ c. show ip protocols

❍ d. show ip-protocols

○

Chapter 6

The correct answer is c. Answer c is correct, because the command will display IP
routing protocols in use. Answer a is incorrect, because this command will dis-
play all layer 3 addresses in use per interface and will not display the routing
protocol in use. Answer b is incorrect, because the command will display net-
works discovered but may not show all remote networks because they may be
restricted by some form like a redistribution access-list. Answer d is incorrect,
because this command displays the IP routing table and indicates how the net-
work was learned.

Question 9

Which of the following routing protocols uses DUAL?

❍ a. RIP

❍ b. RIP II

❍ c. OSPF

❍ d. EIGRP

The correct answer is d. Only EIGRP runs DUAL, or diffusing update algo-
rithm. Answers a, b, and c are incorrect, because RIP, RIP II, and OSPF do not
use DUAL.

Question 10

If the metric calculated by IGRP is 1,000, what would the metric be if EIGRP
was used. Assume no other metric has been adjusted?

❍ a. 1,000

❍ b. 2,000

❍ c. 256

❍ d. 256,000

The correct answer is d. The metric calculated by EIGRP is 256 times that calcu-
lated by IGRP. Therefore, 256*1000=256000. Answers a, b, and c are incorrect,
because they present invalid metrics.

○

Routing and Switching Algorithms

Question 11

What is the default cost for an OSPF router on an Ethernet interface?

❍ a. 1

❍ b. 10

❍ c. 100

❍ d. 1000

The correct answer is b. The OSPF cost is 10

/10

=10, where the default band-

width on an Ethernet interface is 10Mbps. Answers a, c, and d are incorrect,
because they present invalid values.

Question 12

OSPF networks must contain what area?

❍ a. 224.0.0.5

❍ b. 224.0.0.6

❍ c. 0.0.0.0

❍ d. 224.0.0.4

The correct answer is c. OSPF networks must have a backbone area, which can
be designate as 0.0.0.0, or 0. Answers a, b, and d are incorrect, because the an-
swers present IP addresses used in multicasts.

Question 13

All OSPF enabled routers listen to what Class D address?

❍ a. 224.0.0.5

❍ b. 124.0.0.5

❍ c. 124.0.0.10

❍ d. 255.255.255.255

The correct answer is a. OSPF routers listen to packets to the multicast address
224.0.0.5. Answer b is incorrect, because 124.0.0.5 is not a mutlicast address.
Answer c is incorrect, because 124.0.0.10 is not a mutlicast address. Answer d is

○

Chapter 6

incorrect, because broadcasts are sent with a destination address of
255.255.255.255.

Question 14

What is the hop count limit for OSPF?

❍ a. 15

❍ b. 16

❍ c. 512

❍ d. 100

❍ e. No limit

The correct answer is e. OSPF has no hop count like RIP, IGRP, and EIGRP. A
hop count is not needed because link-state routing protocols remove loops and
part of the hop count limit (infinity) is to prevent loops. The metric used by
OSPF is cost. Answers a, b, c, and d are incorrect, and they are provided to trick
you into thinking OSPF has a hop count.

Question 15

What well-known TCP port does BGP use?

❍ a. 178

❍ b. 179

❍ c. 180

❍ d. 181

❍ e. 182

The correct answer is b. BGP uses TCP to peer to other BGP routers, and BGP
runs over TCP port number 179. Answers a, c, d, and e are incorrect, because
they do not represent well-known TCP ports used by BGP.

○

Routing and Switching Algorithms

Question 16

When configuring OSPF, the router ospf <number> command is issued.
What does the Cisco router use the number for?

❍ a. It is the AS number, which must be unique.

❍ b. It is the AS number, which must be same.

❍ c. It is the process ID on the router and locally significant.

❍ d. There is no number.

The correct answer is c. The process ID is an internally used identification pa-
rameter for an OSPF routing process. It is locally assigned and can be any posi-
tive integer. Cisco routers can run multiple instances of OSPF using different
process ID numbers. Answer a is incorrect, because the AS number does not
need to be unique across any domain. Answer b is incorrect, because the AS
number is only locally significant to the router. Answer d is incorrect, because
there is a required parameter when enabling OSPF.

Question 17

What is the correct syntax to peer a Cisco router to a remote BGP router in
AS 1000 with a next hop IP address of 10.72.128.45?

❍ a. neighbor bgp 1000

❍ b. neighbor 10.72.128.45 remote-as 1000

❍ c. neighbor remote-as 1000 10.72.128.45

❍ d. neighbor remote-as 10.72.128.45 1000

The correct answer is b. The IOS syntax to configure a BGP peer is neighbor
<remote ip address> remote-as <remote AS>. Answer a is incorrect, because the
command is an invalid BGP command. Answer c is incorrect, because as is listed
before the remote IP address. Answer d is incorrect, because the command is an
invalid BGP command.

○

Chapter 6

Question 18

Which of the following IP protocols requires an autonomous system num-
ber? [Choose the two best answers]

❑ a. OSPF

❑ b. IGRP

❑ c. RIP II

❑ d. EIGRP

The correct answers are b and d. Answer b and d are correct, because IGRP and
EIGRP require an AS number. Answer a is incorrect, because OSPF uses a pro-
cess ID that is only locally significant. Answer c is incorrect, because RIP II does
not use an autonomous system ID number.

Question 19

What IOS command will display an OSPF database?

❍ a. show ip ospf

❍ b. show ip route

❍ c. show ip ospf database

❍ d. show ip ospf-database

The correct answer is c. The syntax used to display the OSPF database, which
also identifies the router ID, is show ip ospf database. Answers a, b, and d are
incorrect, because they present invalid IOS commands.

Question 20

To start BGP in AS 2000, what IOS command needs to be issued?

❍ a. router bgp 200

❍ b. router bgp-2000

❍ c. router 2000 bgp

❍ d. router bgp 2000

○

Routing and Switching Algorithms

The correct answer is d. To start BGP, you issue the router BGP <autonomous
system> IOS command. Therefore, only answer d is correct. You must be able to
recognize the correct syntax to start any of the IP routing protocols discussed in this
chapter. Answers a, b, and c are incorrect, because they provide invalid commands.

○

Chapter 6

Need to Know More?

Doyle, Jeff: Routing TCPIP. Macmillan Publication, Indianapolis, IN,
1998. ISBN 1-57870-041-8. Chapters 13 and 14 detail route filtering
and redistribution. There are numerous quality examples.

Halabi, Bassam: Internet Routing Architectures. Macmillan Publishing,
Indianapolis, IN, 1997. ISBN 1-562-0565-22. The introduction to
BGP in Chapter 4 will enhance your understanding of BGP.

Thomas II, Thomas M.: OFPF Network Designs Solutions. Macmillan
Publishing, Indianapolis, IN, 1998. ISBN 1-578-7004-69. Chapter 4
provides a good explanation OSPF at a beginner level, and then moves
onto routing and design. This book is highly recommended for extra
study the Lab portion of the CCIE exam.

Cisco Systems: Cisco IOS 12 Solutions for Network Protocols, Volume I.
Macmillan Publishing, Indianapolis, IN, 1997. ISBN 1-578-7015-46.
Part II of the book describes all the IP routing protocols describe here
in more depth with quality examples.

www.cisco.com enables you to search for “ospf design guide” and “bgp
design guide.” Both guides are excellent guides for both the technical
and practical CCIE exams. There are very good examples to compli-
ment the ones described in this chapter.

http://cco/univercd/cc/td/doc/cisintwk/ics/index.htmprovides
internetworking case studies. This site has some excellent examples of
BGP design and route redistribution between RIP and OSPF.

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