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  • Print publication year: 2004
  • Online publication date: June 2012

3 - Bridges, LANs and the Cisco IOS



I think that I shall never see

A graph more lovely than a tree.

A tree whose crucial property

Is loop-free connectivity.

A tree that must be sure to span

So packets can reach every LAN.

First, the root must be selected.

By ID, it is elected.

Least-cost paths from root are traced.

In the tree, these paths are placed.

A mesh is made by folks like me,

Then bridges find a spanning tree.

Radia Perlman


The Cisco Internet Operating System software.

Configuring a Cisco router.

Transparent bridge configuration and operation.

The spanning tree algorithm.

Ethernet bridges

Use of bridges

Bridges are link layer devices. As illustrated in Fig. 0.3, when two network segments with different link and physical layer protocols are connected, the bridge performs a two-way translation of the protocols. The data section of a transit frame is extracted and re-encapsulated in the frame format used by the next-hop network segment.

However, there are several reasons to use bridges to connect networks even with identical protocols, rather than using a large network without bridges. First, network segments could be far away from each other but still work within the same logical network. In this case, two remote network segments can be linked by two bridges via a point-to-point wide area link. Second, there is a limit on the maximum length of the shared medium in a local network.