Bridges and Switches

To improve performance, LANs are usually divided into smaller multiple LANs. These LANs are then interconnected by a LAN switch or by a bridge, as shown in Figure C-3.

Figure C-3 LANs Are Split into Many Smaller LANs, Using Switches or Bridges to Improve Performance

Switches and bridges have some "smarts." When devices are connected via a switch or a bridge, a device will only hear the following:

• Everything that the other devices on its port send

• Any information from devices on other ports that was meant for everyone (a broadcast)

• Any information from devices on other ports that was meant for devices on its port

A device connected to a switch or bridge will not hear any of the information meant just for devices on other ports of the switch.

Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types operate at the data link layer, they are not required to examine upper-layer information. This means that they can rapidly forward traffic representing any network layer protocol. It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic between two or more networks.

By dividing large networks into self-contained units, bridges and switches provide several advantages. Because only a certain percentage of traffic is forwarded, a bridge or switch diminishes the traffic experienced by devices on all connected segments. Bridges and switches extend the effective length of a LAN, permitting the attachment of distant stations that were not previously permitted.

Switches and bridges may communicate with each other by using a bridging protocol. Several types of bridging protocols are supported by Cisco routers (acting as bridges) and switches, including the following:

• Transparent bridging—Found primarily in Ethernet environments.

• Source-route bridging (SRB)—Found primarily in Token Ring environments.

• Translational bridging—Translates from Ethernet bridging to Token Ring bridging.

Figure C-3 LANs Are Split into Many Smaller LANs, Using Switches or Bridges to Improve Performance

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