System information

CHAPTER
Troubleshooting Transparent Bridging Environments 20-421
20
Troubleshooting Transparent Bridging
Environments
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
Transparent Bridging Technology Basics
Transparent bridges are so named because their presence and operation are transparent to network
hosts. When transparent bridges are powered on, they learn the network’s topology by analyzing the
source address of incoming frames from all attached networks. If, for example, a bridge sees a frame
arrive on line 1 from Host A, the bridge concludes that Host A can be reached through the network
connected to line 1. Through this process, transparent bridges build a table such as the one in
Figure 20-1.
Figure 20-1 A Transparent Bridging Table
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 are also flooded in this way.
Transparent bridges successfully isolate intrasegment traffic, thereby reducing the traffic seen on
each individual segment. This usually improves network response times as seen by the user. The
extent to which traffic is reduced and response times are improved depends on the volume of
intersegment traffic relative to the total traffic as well as the volume of broadcast and multicast traffic.
Host address
15
17
12
13
18
9
14
Network number
1
1
2
2
1
1
3