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From Hubs to VLANs

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Using Hubs

• Layer 1 devices
• Inexpensive
• In one port, out the others
• One collision domain
• One broadcast domain

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This is fine for small workgroups, but does not

scale well for larger workgroups or heavy
traffic.

Hub 1

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

172.30.1.24

255.255.255.0

Single Hub

Ÿ

One Network (IP Network Address - usually)

ŸOne Collision Domain

ŸOne Broadcast Domain

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•

What if the computers were on two different
subnets? Could they communicate within their own
subnet? Yes Between subnets? No, need a
router.

Single Hub - Two subnets

Ÿ Two subnets

Ÿ One Collision Domain

Ÿ One Broadcast Domain

Hub 1

172.30.1.21

255.255.255.0

172.30.1.22

172.30.1.23

172.30.1.24

255.255.255.0

255.255.255.0

255.255.255.0

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• Same issues as before, with more of an impact on

the network.

All Hubs

Ÿ One Network Address

Ÿ One Collision Domain

Ÿ One Broadcast Domain

Hub 1

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Hub 2

172.30.1.24

255.255.255.0

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

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Using Switches

• Layer 2 devices
• Moderate expense for common access

switches, but can be very expensive.

• Layer 2 filtering based on Destination MAC

addresses and Source Address Table

• One collision domain per port
• One broadcast domain

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Switch and Hub Network

Ÿ One Network

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One for the entire Hub

Ÿ One Broadcast Domain

Hub

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Switch

172.30.1.24

255.255.255.0

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

Two virtual circuits: (complete SAT tables)
Data traffic from 172.30.1.24 to 172.30.1.25
and from 172.30.1.26 to 172.30.1.27

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Switch and Hub Network

Ÿ One Network

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One for the entire Hub

Ÿ One Broadcast Domain

Hub

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Switch

172.30.1.24

255.255.255.0

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

As opposed to the Hub:
Data traffic from 172.30.1.21 to 172.30.1.22
and from 172.30.1.23 to 172.30.1.24

Collision!

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Switch and Hub Network

Ÿ One Network

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One for the entire Hub

Ÿ One Broadcast Domain

Hub

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Switch

172.30.1.24

255.255.255.0

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

Collisions and Switches:
What happens when two devices on a switch, send

data to another device on the switch.
172.30.1.24 to 172.30.1.25 and 172.30.1.26 to

172.30.1.25

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Switch and Hub Network

Ÿ One Network

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One for the entire Hub

Ÿ One Broadcast Domain

Hub

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Switch

172.30.1.24

255.255.255.0

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

The switch keeps the frames in buffer memory, and

queues the traffic for the host 172.30.1.25. This

means that the sending hosts do not know about the

collisions and do not have to re-send the frames.

Frames
in
buffer

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Other Switching Features

Review
• Asymmetric ports: 10 Mbps and 100 Mbps
• Full-duplex ports
• Cut-through versus Store-and-Forward

switching

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Ports between switches and server ports are good

candidates for higher bandwidth ports (100 Mbps) and

full-duplex ports.

All Switched Network

Ÿ One Network

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One Broadcast Domain

Switch 1

172.30.1.21

255.255.255.0

172.30.1.22

255.255.255.0

172.30.1.23

255.255.255.0

Switch 2

172.30.1.25

255.255.255.0

172.30.1.26

255.255.255.0

172.30.1.27

255.255.255.0

172.30.1.28

255.255.255.0

172.30.1.24

255.255.255.0

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Introducing Multiple

Subnets/Networks without

Routers

• Switches are Layer 2 devices
• Router are Layer 3 devices
• Data between subnets/networks must pass

through a router.

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All Switched Network - Two Networks

Ÿ Two Subnets

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One Broadcast Domain

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

Switch 2

172.30.1.25

255.255.255.0

172.30.2.14

255.255.255.0

172.30.1.27

255.255.255.0

172.30.2.16

255.255.255.0

172.30.2.12

255.255.255.0

A Switched Network with two subnets:
What are the issues? Can data travel within the

subnet? Yes Can data travel between subnets? No,

need a router! What is the impact of a layer 2

broadcast, like an ARP Request?

ARP
Request

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All Switched Network - Two Networks

Ÿ Two Subnets

Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ One Broadcast Domain

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

Switch 2

172.30.1.25

255.255.255.0

172.30.2.14

255.255.255.0

172.30.1.27

255.255.255.0

172.30.2.16

255.255.255.0

172.30.2.12

255.255.255.0

All devices see the ARP Request. One broadcast

domain means the switches flood all broadcast out all

ports, except the incoming port. Switches have no idea

of the layer 3 information contained in the ARP

Request. This consumes bandwidth on the network and

processing cycles on the hosts.

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One Solution:
Physically separate the subnets. But still no data can

travel between the subnets. How can we get the data

to travel between the two subnets?

Two Switched Networks

Ÿ Two Subnets
Ÿ Several Collision Domains

Ÿ One per switch port

Ÿ Two Broadcast Domain

Switch 1

172.30.1.21

255.255.255.0

172.30.1.23

255.255.255.0

172.30.1.25

255.255.255.0

Switch 2

172.30.2.10

255.255.255.0

172.30.2.12

255.255.255.0

172.30.2.14

255.255.255.0

172.30.2.16

255.255.255.0

172.30.1.26

255.255.255.0

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Introducing Multiple

Subnets/Networks with Routers

• Switches are Layer 2 devices
• Router are Layer 3 devices
• Data between subnets/networks must

pass through a router.

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Routed Networks

Ÿ Two Subnets

Ÿ Several Collision Domains

Ÿ One per switch port

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.1.23

255.255.255.0

172.30.1.25

255.255.255.0

Switch 2

172.30.2.10

255.255.255.0

172.30.2.12

255.255.255.0

172.30.2.14

255.255.255.0

172.30.2.16

255.255.255.0

172.30.1.26

255.255.255.0

Router

172.30.1.1

255.255.255.0

172.30.2.1

255.255.255.0

Routed Network:
Two separate broadcast domains, because the

router will not forward the layer 2 broadcasts such

as ARP Requests.

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Switches with multiple subnets

• So far this should have been a review.
• Let’s see what happens when we have two

subnets on a single switch and we want to
route between the two subnets.

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Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

172.30.2.1 sec

255.255.255.0

Router-on-a-stick:
When a single interface is used to route between

subnets or networks, this is known as a router-on-a-

stick. To assign multiple ip addresses to the same

interface, secondary addresses or subinterfaces are

used.

interface e 0
ip address 172.30.1.1

255.255.255.0
ip address 172.30.2.1

255.255.255.0 secondary

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Router-on-a-stick

Advantages
• Useful when there are limited Ethernet

interfaces on the router.

Disadvantage
• Because a single link is used to connect

multiple subnets, one link is having to carry
the traffic for multiple subnets.

• Be sure this is link can handle the traffic.

You may wish to use a high-speed link (100
Mbps) and full-duplex.

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Gotcha’s

1. Remember to have the proper default

gateway set for each host.

• 172.30.1.0 hosts - default gateway is

172.30.1.1

• 172.30.2.0 hosts - default gateway is

172.30.2.1

2. The router must still route between

subnets, so you must include:

Router (config)# router rip
Router (config-router)# network

172.30.0.0

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Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

255.255.255.0

172.30.2.1

255.255.255.0

Multiple interfaces:
Two Ethernet router ports may be used instead of one.

However this may be difficult if you do not have

enough Ethernet ports on your router.

E
0

E
1

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One switch two subnets:
Good News: Data can travel between subnets and

we have two separate broadcast domains. Bad

News: Hosts are on different subnets but on a

single layer 2 broadcast domain.

Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

172.30.2.1 sec

255.255.255.0

ARP
Request

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An ARP Request from 172.30.1.21 for 172.30.1.23 will

still be seen by all hosts on the switch. The switch is a

layer 2 device and will flood broadcast traffic out all

ports, except the incoming port.

Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

172.30.2.1 sec

255.255.255.0

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Introducing VLANs

• VLANs create separate broadcast domains
• Routers are needed to pass information

between different VLANs

• VLANs are not necessary to have separate

subnets on a switched network, but as we
will see they give us more advantages when
it comes to things like data link (layer 2)
broadcasts.

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Two VLANs

Ÿ Two Subnets

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

Layer 2 broadcast control:
An ARP Request from 172.30.1.21 for 172.30.1.23 will

only be seen by hosts on that VLAN. The switch will

flood broadcast traffic out only those ports belonging

to that particular VLAN, in this case VLAN 1.

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Port-centric VLAN Switches

Remember, as the Network Administrator, it is your
job to assign switch ports to the proper VLAN. This
assignment is only done at the switch and not at the
host. Note: The following diagrams show the VLAN
below the host, but it is actually assigned within the
switch.

1 2 3 4 5 6 .
1 2 1 2 2 1 .

Port

VLAN

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Catalyst 1900 - VLAN Membership

Configuration

[M] Membership type

[V] VLAN assignment

[R] Reconfirm dynamic membership

[X] Exit to

previous menu
Enter Selection:

Port

VLAN

Membership Type

1

1

Static

2

2

Static

3

1

Static

4

2

Static

5

2

Static

6

1

Static

7

1

Static

8

1

Static

9

1

Static

10

1

Static

11

1

Static

12

2

Static

AUI

1

Static

A

1

Static

B

1

Static

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Layer 2 broadcast control:
Without VLANs, the ARP Request would be seen by

all hosts. Again, consuming unnecessary network

bandwidth and host processing cycles.

No VLANs

Ÿ

Same as a single VLAN

Ÿ

Two Subnets

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Switch 1

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Two VLANs

Ÿ

Two Subnets

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

With VLANs:
Data will only travel within the VLAN. Remember that

switches are Layer 2 devices and they can only pass

traffic within the VLAN.

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1 2 3 4 5 6 .
1 2 1 2 2 1 .

Port

VLAN

Switch Port: VLAN ID

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Two VLANs

Ÿ

Two Subnets

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

With VLANs:
A switch cannot route data between different VLANs.

Example: Data from 172.30.1.21 to 172.30.2.12

X

Switch Port: VLAN

ID

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Gotcha’s
1. Remember that VLAN IDs (numbers) are

assigned to the switch port and not to the host.
(Port-centric VLAN switches)

2. Be sure to have all of the hosts on the same

subnet belong to the same VLAN, or you will
have problems.

Hosts on subnet 172.30.1.0/24 - VLAN 1
Hosts on subnet 172.30.2.0/24 - VLAN 2
etc.

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Routing and VLANs

• In the previous example data could travel within

the VLAN, but not between VLANs.

• Just like subnets, a router is needed to route

information between different VLANs.

• The advantage is the switch propagates broadcast

traffic only within the VLAN.

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VLANs

Ÿ Two Subnets

ŸCommunication between VLANs

Ÿ NOTE

: VLANs assigned only to the

ports

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

Router

172.30.1.1

255.255.255.0

VLAN 1

172.30.2.1

255.255.255.0

VLAN 2

Data between VLANs is routed through the router.

Data from 172.30.1.21 to 172.30.2.12

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Gotcha’s

1. Remember to have the proper default gateway

set for each host.

• 172.30.1.0 hosts - default gateway is 172.30.1.1
• 172.30.2.0 hosts - default gateway is 172.30.2.1

2. The router must still route between subnets, so you

must include:

Router (config)# router rip
Router (config-router)# network 172.30.0.0
3. The switch ports to the router must have the

corresponding VLAN ID to that subnet.

Switch port to 172.30.1.1 must be on VLAN 1
Switch port to 172.30.2.1 must be on VLAN 2

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Switch Port: VLAN ID

Router

172.30.1.1

255.255.255.0

(VLAN 1)

172.30.2.1

255.255.255.0

(VLAN 2)

(VLAN ID not set at
router.)

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So, what’s the difference?

• One of the main differences between

subnets with VLANs and subnets without
VLANs on switched networks, is that
VLANs offer layer 2 broadcast control.

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Here is an ARP Request example without VLANs

.

Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

255.255.255.0

172.30.2.1

255.255.255.0

ARP
Request

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Here is an ARP Request example with VLANs. Notice

that the broadcast is isolated only to the VLAN that it

came from, in this case VLAN 1.

VLANs

Ÿ Two Subnets

ŸCommunication between VLANs

Ÿ NOTE

: VLANs assigned only to the

ports

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

Router

172.30.1.1

255.255.255.0

VLAN 1

172.30.2.1

255.255.255.0

VLAN 2

ARP
Request

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Cisco Networking Academy Program

© Cisco Systems, Inc.
2000

• Can I use the Router-on-a-stick method

with multiple VLANs?

• Can you remind me what Router-on-a-stick

is?

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Cisco Networking Academy Program

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Routed Networks

Ÿ Two Subnets

ŸCommunication between subnets

Switch 1

172.30.1.21

255.255.255.0

172.30.2.10

255.255.255.0

172.30.1.23

255.255.255.0

172.30.2.12

255.255.255.0

Router

172.30.1.1

172.30.2.1 sec

255.255.255.0

What is Router-on-a-stick?
When a single interface is used to route between

subnets or networks, this is know as a router-on-a-stick.

To assign multiple ip addresses to the same interface,

secondary addresses or subinterfaces are used.

interface e 0
ip address 172.30.1.1

255.255.255.0
ip address 172.30.2.1

255.255.255.0 secondary

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Cisco Networking Academy Program

© Cisco Systems, Inc.
2000

With Router-on-a-stick, ISL or 802.1Q trunking is

needed. We will talk about tagging and trunking in

the next section.

VLANs

Ÿ Two Subnets

Ÿ

Communication between VLANs using trunking

Ÿ NOTE

: VLANs assigned only to the ports

Switch 1

172.30.1.21

255.255.255.0

VLAN 1

172.30.2.10

255.255.255.0

VLAN 2

172.30.1.23

255.255.255.0

VLAN 1

172.30.2.12

255.255.255.0

VLAN 2

Router

172.30.1.1

172.30.2.1 secondary

255.255.255.0

Trunking ISLor 802.1Q

Trunking ISL or 802.1Q

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Cisco Networking Academy Program

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2000

Non-tagging Switches

• Lets first see how multiple VLANs are

interconnected using switches that do not
have the tagging capability.

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Cisco Networking Academy Program

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2000

100BaseT Ports

Port 1 = VLAN 1 & Port 2 =
VLAN 2

Moe

Larry

VLAN 1: Port 1 on switch Moe is connected
to Port 1 on Switch Larry.

VLAN 2: Port 2 on switch Moe is connected
to Port 2 on Switch Larry.

Non-tagging Switches

For each VLAN, there must be a link between the
two switches. One link per VLAN. Be sure the
switch ports on the switches are configured for the
proper VLAN.

1 2

1 2

Port 1 = VLAN 1 & Port 2 =
VLAN 2

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Cisco Networking Academy Program

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Advantages

• Each VLAN gets its own dedicated link with

its own bandwidth.

Disadvantages

• This requires a separate link for each VLAN.

There may not be enough ports on the
switch to accommodate a lot of different
VLANs.