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C H A P T E R23
Chapter Goals
• Understand transparent bridge processes of learning, filtering, forwarding, and flooding.
• Explain the purpose of the spanning-tree algorithm.
• Describe the bridge and port modes in a spanning-tree network.
Transparent Bridging
Transparent bridges were first developed at Digital Equipment Corporation (Digital) in
the early 1980s. Digital submitted its work to the Institute of Electrical and Electronic Engineers (IEEE),
which incorporated the work into the IEEE 802.1 standard. Transparent bridges are very popular in
Ethernet/IEEE 802.3 networks. This chapter provides an overview of transparent bridging’s handling of
traffic and protocol components.
Transparent Bridging Operation
Transparent bridges are so named because their presence and operation are transparent to network hosts.
When transparent bridges are powered on, they learn the workstation locations by analyzing the source
address of incoming frames from all attached networks. For example, if a bridge sees a frame arrive on
port 1 from Host A, the bridge concludes that Host A can be reached through the segment connected to
port 1. Through this process, transparent bridges build a table (the learning process), such as the one in
Figure 23-1.
23-1
rnetworking Technologies Handbook
Chapter 23
Transparent Bridging
Transparent Bridging Operation
Figure 23-1 Transparent Bridges Build a Table That Determines a Host’s Accessibility
The bridge uses its table as the basis for traffic forwarding. When a frame is received on one of the
bridge’s interfaces, the bridge looks up the frame’s destination address in its internal table. If the table
contains an association between the destination address and any of the bridge’s ports aside from the one
on which the frame was received, the frame is forwarded out the indicated port. If no association is
found, the frame is flooded to all ports except the inbound port. Broadcasts and multicasts also are
flooded in this way.
Transparent bridges successfully iso