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INTRODUCTION
1-6
1.5.2 Primary and Redundant Data Paths
To ensure the integrity of the network, primary data paths through
designated bridges, and redundant data paths through alternative bridges
are determined in each LAN or loop configuration in the network. A LAN
segment’s designated bridge is the bridge in a LAN or loop configuration
with the lowest cost path of transmission from that LAN segment to the
root bridge. The lowest cost path can be translated in a general way to
mean the data transmission path to the root that is most economical.
The bridges in the network send out BPDUs to the other bridges so that
they can all learn which is the bridge in the network, or in a potential loop
configuration, with the lowest cost path to the root. The bridges can then
determine their own priorities and roles as designated or redundant
bridges. Redundant bridges are “blocked” from forwarding packets, but
they continue to receive topology information.
Figure 1-2 provides a sample illustration of how a potential data loop is
prevented in a simple multibridge environment. A data loop exists
Figure 1-2 Multiple Bridging Environments
T T
T T
NODE A
BRIDGE 2
NODE B
T T
BRIDGE 1
BRIDGE 3
NB20E OR 25E
NB20E OR 25E
NB20E OR 25E