2-1

Chapter 2

Bridging
About bridging methods; viewing and managing bridging interfaces; using the Bridge Status window;
viewing bridge statistics; using Spanning Tree; using the Filtering Database; configuring duplex
modes; using the Interface Configuration window; and setting Bridge Translation

Bridging Basics

Bridges are used in local area networks to connect two or more network segments
and to control the ßow of packets between the segments. Ideally, bridges forward
packets to another network segment only when necessary.
Bridges are also used to increase the fault tolerance in a local area network by
creating redundant bridge paths between network segments. In the event of a
bridge or bridge segment failure, an alternate bridge path will be available to
network trafÞc, without signiÞcant interruption to its ßow.
The method a bridge uses to forward packets, choose a bridge path, and ensure
that a sending stationÕs messages take only one bridge path depends on the
bridgeÕs type: Transparent (generally used in Ethernet or FDDI environments) or
Source Routing (generally used in Token Ring environments), source
routing-transparent, or source route-transparentÑthe two latter being
combinations that are found in a mixed network environment.
About Transparent Bridging
Transparent bridges are most common in Ethernet networks. Individual
Transparent bridges monitor packet trafÞc on attached network segments to learn
where end stations reside in relation to each segment by mapping the Source
Address of each received frame to the port (and segment) it was detected on. This
information gets stored in the bridgeÕs Filtering Database.
When in the Forwarding state, the bridge compares a packetÕs destination address
to the information in the Filtering Database to determine if the packet should be
forwarded to another network segment or Þltered (i.e., not forwarded). A bridge