Spanning Tree Customization

The most important decision you can make when designing your Spanning-Tree topology is the placement of the Root Bridge. Other decisions, such as the exact loop-free path structure, will occur automatically as a result of the STA. Occasionally, the path may need additional tuning, but only under special circumstances and after careful consideration.

Recall the sequence of four criteria that STP uses to choose a path:

1 Lowest Bridge ID

2 Lowest Root Path Cost

3 Lowest Sender Bridge ID

4 Lowest Port ID

The previous section discussed how to tune the Bridge ID of a switch to place the Root Bridge in a network. This technique can be used to force a switch to have the lowest Bridge ID and also to influence the sending Bridge ID of other switches (Lowest Bridge ID and Lowest Sender Bridge ID). However, only the automatic STP computation has been discussed, using the default switch port costs to make specific path decisions.

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