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AT-S88 Management Software User’s Guide

Port Priority

If two paths have the same port cost, the bridges must 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 range for port priority is 0 to 240. As with bridge priority, this range is

broken into increments, in this case multiples of 16. Table 8 lists the

values and increments. The default value is 128.

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-S88 management

Table 8. Port Priority Value Increments

Port

Priority

Port

Priority

0 128

16 144

32 160

48 176

64 192

80 208

96 224

112 240

1 2 ... 102 103 104 105 106 107 108 109 110 111 112 ... 157 158

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