Allied Telesis AT-S62 - Manual (Page 334)

Chapter 18: STP and RSTP
Section IV: Spanning Tree Protocols 334
Port Priority
If two paths have the same cost, the bridges must choose between them
to select a preferred path. In some instances this can involve the use of
the port priority parameter. This parameter is used as a tie-breaker when
two paths have the same cost. The lower the value, the higher the
priority given to the port.
The range for port priority is 0 to 240. As with bridge priority, this range
is broken into increments, in this case multiples of 16. To select a port
priority for a port, you enter the increment of the desired value. Table 17
lists the values and increments. The default value is 128, which is
increment 8.
Forwarding Delay and Topology Changes
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports. However, a
change in a port state is not activated immediately.
It might take time for the root bridge to notify all bridges that a topology
change has occurred, especially if it is a large network. If a topology
change is made before all bridges have been notified, a temporary data
loop could occur, and that could adversely impact network
performance.
Table 17 Port Priority Value Increments
Increment
Port
Priority
Increment
Port
Priority
0 0 8 128
1 16 9 144
2 32 10 160
3 48 11 176
4 64 12 192
5 80 13 208
6 96 14 224
7 112 15 240