Allied Telesis AT-S62 - Manual (Page 488)

Chapter 22: Spanning Tree and Rapid Spanning Tree Protocols
488 Section IV: Spanning Tree Protocols
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.
To forestall the formation of temporary data loops during topology
changes, a port designated to change from blocking to forwarding passes
through two additional states—listening and learning—before it begins to
forward frames. The amount of time a port spends in these states is set by
the forwarding delay value. This value states the amount of time that a
port spends in the listening and learning states prior to changing to the
forwarding state.
The forwarding delay value is adjustable in the AT-S62 management
software. The appropriate value for this parameter depends on a number
of variables, the size of your network being a primary factor. For large
networks, you should specify a value large enough to allow the root bridge
sufficient time to propagate a topology change throughout the entire
network. For small networks, you should not specify a value so large that a
topology change is unnecessarily delayed, which could result in the delay
or loss of some network traffic.
Table 16. 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