Allied Telesis AT-S63 - Manual (Page 254)

Chapter 16: STP and RSTP
254 Section II: Advanced Features
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.
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-S63 management
software. The appropriate value for this parameter depends on a
number of variables; the size of your network is a primary factor. For
large networks, you should specify a value large enough to allow the
Table 17. Port Priority Value Increments
Increment
Bridge
Priority
Increment
Bridge
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