3Com PathBuilder S200 Series, Owner'S Manual

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Bridging 93
T0008-16F Release 5.2M
Spanning Tree Protocol Entity (STPE)
Other
Considerations for
Selecting Links
There are two final considerations when selecting links on the basis of reported cost,
when the costs and indicated root bridge on different links are the same. The first is
the case where, for example, B4 receives a message on link 8 from B3 designating
B1 as the root bridge with a cost of 10608 to the root. At the same time, B4 receives
a message on link 8 from B2 designating B1 as the root bridge with a cost of 7072.
In this case, B4 will select link 7 as the root link because the spanning tree algorithm
dictates that if more than one message has the same root bridge indicated, at the same
cost to the root, then the message with the higher priority-reporting adjacent should
be given priority. In this case, B2 has been configured to have a higher priority
(lower Bridge ID) than B3, so B4 selects link 7 as its root link.
The second consideration occurs when two links on the same bridge are receiving
messages from the same adjacent bridge, and the messages report the same root
bridge and the same cost to the root bridge. This could occur if there were two links
between B3 and B4. The bridge will choose the link with the lower bridge link
priority. The priority for the link is a 2-byte number formed by concatenating the
value of the parameter STPE Priority with the link number.
Figure 46. Two Bridge Links Between B3 and B4
For example, B3 sends messages to B4 that the root bridge is B1, and that the cost to
the root is 10608 (3536+3536). To cause bridge 4 to favor link 11 over link 8,
configure:
link 8: STPE Priority = 128 (80 hex)
link 11: STPE Priority = 64 (40 hex)
In this example, should a link between bridge 3 and bridge 4 be necessary for a
spanning tree, bridge 4 will favor link 11 and remove link 8 from the tree.
Bridge 3 Bridge 4
6 7
1
9
10
1
8
LAN 3
LAN 4
11