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94 Bridging
Spanning Tree Protocol Entity (STPE)
The same priority mechanisms that determine the root link are also applied in
determining which links become designated links (a root link is never a designated
link). The designated link is the link that is responsible for issuing the bridge
messages when more than one link is involved in a network. For example, in the
network in Figure 43, Bridge 5 and bridge 6 are connected to LAN 5 and will both
issue each other spanning tree messages until they determine which one of them is
the designated bridge for LAN 5. Once determined, the designated bridge issues
bridge messages and the other bridge only listens (unless it has received another
message from another link that would make its link the designated bridge; such a
message must have higher priority than the one it receives from its designated
adjacent). Another case where there is contention and resolution to a designated
bridge is link 8 between bridge 3 and bridge 4.
To determine which link becomes designated, the same set of priority parameters are
used as in determining the root link. In this case, the designated link is the link
issuing the message that:
• Identifies the root bridge with the lowest numerical bridge ID
• Has the lowest cost to the root (assuming there is a tie in reporting the root
bridge)
• Identifies itself with a higher priority ID (assuming there is a tie in reporting
the root bridge and the cost to the root bridge)
• Has the higher priority link (assuming all of the above are tied)
In this network, bridge 5 link 1 is the designated link for LAN 5 because between
bridge 5 link 1 and bridge 6 link 1, bridge 5 link 1 generates a message with a lower
cost to the root than bridge 6 link 1 (they both have the same root). Between bridge 3
and bridge 4 on link 8, bridge 3 link 8 becomes the designated link because it has a
lower cost to the link.
Links that are not root links or are not designated links are not part of the spanning
tree. Links that are not on the spanning tree do not forward data packets (or spanning
tree explorer frames in source route bridging). However, they are constantly
receiving bridge messages on these links from the designated bridges and comparing
these messages to those they originate. This action allows the bridge to detect
failures and adjust the spanning tree, should this become necessary.