Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Bridge A bridge is a network device that operates at Layer 2 of the OSI Reference Model. It connects to two or more LAN segments (collision domains) and maintains a table describing which MAC addresses (devices) are connected to each bridge port. Most bridges require minimal configuration, they learn where stations reside automatically. From a bridge’s perspective (Data Link — OSI Layer 2) there are only two type of packets — unicast and broadcast (see Exhibit 3-7-3). For unicast packets, the bridge attempts to match the destination MAC address in the frame header to one of its table entries. If the packet matches an entry AND the destination MAC is the same port on which the packet was received, the packet is dropped. If the packet matches an entry AND the destination MAC is on a different port than the one the packet was received, then the packet is forwarded out the destination port. Finally, if the packet does not match an entry in the table, then the packet is forwarded out all ports — similar to broadcast packets. Exhibit 3-7-3.  Bridged Environment Bridges also forward broadcast packets. When a broadcast packet is received on a given bridge port, the packet is copied and propagated out all other bridge ports. This function can create broadcast storms in multi-bridge networks. Router A router is a network device that operates at Layer 3 (Network) of the OSI Reference Model. It connects two or more networks and maintains tables indicating which networks can be reached through each port. Most routers require careful configuration — erroneous information entered into a router’s configuration can create chaos on any network. Routers interpret packets based upon information within the Network portion of a packet. They examine the protocol header to determine the destination network and perform a table lookup to determine the port with the most efficient path to the destination. The packet is then forwarded out the correct port (see Exhibit 3-7-4). Exhibit 3-7-4.  Routed Environment Unlike bridges, routers do not process every packet on each connected network. Only packets which need to travel from one network to another must traverse the router. In most cases, the network protocols help the sending machine determine if the destination resides on a local or remote network. If on a remote network, then the sending station will send the packet directly to the router. Furthermore, unless specifically configured to do so, routers will not forward broadcast packets. Typically broadcast domains are bounded by router ports. Switch Switches function exactly like bridges, with only a few differences. Typically switches contain only LAN interfaces, while bridges may also support WAN connections. Switches may have much higher port density (e.g., 192 Ethernet ports in a single switch is not uncommon) than bridges. Switches are generally designed around an ASIC architecture (rather than a bridge’s CPU oriented design) to provide much high levels of performance (see Exhibit 3-7-5). Exhibit 3-7-5.  Switch Environment Lastly, switches often have advanced features (such as VLANs) which allow more flexible configurations in LAN environments. Layer 3 Switch There is a lot of confusion regarding this new class of network devices since Layer 3 devices have always been called routers. For the purpose of this chapter, we will consider Layer 3 switches to behave like high-speed routers with traditional switch functions. Virtual LAN (VLAN) Creating virtual LANs (VLANs) is a feature available on most switches sold today. A VLAN is a tool for creating broadcast domain boundaries within a set of switches. VLANs allow network administrators to control which devices are members of a particular broadcast domain. Some switches allow designers to select VLAN members based upon a variety of attributes such as: port, MAC address, protocol type. Many switches employ VLAN tagging (via 802.1Q or a proprietary protocol) so that multiple switches will be able to determine and share VLAN membership information (see Exhibit 3-7-6). Exhibit 3-7-6.  VLAN Environment VLAN Standards Until recently, no standards have existed for VLANs and, for the most part, this has not posed a problem. Switch vendors implement VLAN tagging for packets traversing multiple switches differently, and as long as a network is composed of switches from a single vendor all is well. However, when a new vendor is introduced to a switch network, a standard tagging mechanism is required to ensure that all devices understand VLAN membership. This need has been addressed by 802.1Q. IEEE 802.1Q provides a standard mechanism for identifying which VLAN each packet belongs to. Starting in late 1997, most Ethernet NICs and Ethernet switch vendors will support this standard. VLANs can be a powerful tool when designing networks, but without proper understanding of network protocols and topology, their use can create administrative and support difficulties — to say the least. The remainder of this chapter will discuss how VLANs, in conjunction with routing and switching, can best be implemented in modern networks. WHEN TO USE SWITCHING In most cases, switching is the simplest way to increase performance within a particular broadcast domain. Congested segments experiencing high collision rates can be efficiently segmented with minimal configuration changes. Installing switches typically does not require major changes to network designs, nor do end stations need to be modified. The network (broadcast domain) is simply more segmented and, therefore, provides more available bandwidth to network devices. (See Exhibit 3-7-7.) Exhibit 3-7-7.  Switching Boosts Performance Risk Factors — Switching Although implementing switching does provide a performance boost to congested networks, other factors must be weighed before determining the extent to which switching should be deployed. Management/Control Extensive deployment of switching, without proper consideration for the size of (number of devices in) broadcast domains can create configurations that allow broadcast storms. Extensive broadcasts can congest the network by utilizing available bandwidth and preventing user data from accessing network resources. And since broadcast packets are processed by every station on a broadcast domain, workstations and servers must interrupt activities to inspect each broadcast packet — further slowing performance. (See Exhibit 3-7-8.) Exhibit 3-7-8.  Broadcasts in Switched Environment Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.