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Network Warrior, 2nd Edition by Gary A. Donahue

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Designing to Prevent Spanning Tree Problems

Proper design can help minimize spanning tree problems. One of the simplest ways to help keep trouble to a minimum is to document and know your network. If you have to figure out how your network operates when there’s a problem, the problem may last longer than your job.

Use Routing Instead of Switching for Redundancy

The saying used to be, “Switch when you can and route when you have to.” But in today’s world of fast Layer-3 switching, this mantra no longer holds. With Layer-3 switches, you can route at switching speeds.

For many people, Layer-3 redundancy is easier to understand than Layer-2 redundancy. As long as the business needs are met and the end result is the same, using routing to solve your redundancy concerns is perfectly acceptable.

Note

If you decide to use routing instead of switching, don’t turn off spanning tree. Spanning tree will still protect against loops you might have missed. If you’re using switches—even Layer-3 switches—spanning tree can be a lifesaver if someone plugs in a switch where it doesn’t belong.

Always Configure the Root Bridge

Don’t let spanning tree elect the root bridge dynamically. Decide which switch in your network should be the root, and configure it with a bridge priority of 1. If you let the switches decide, not only may they choose one that doesn’t make sense, but switches added later may assume the role of root bridge. This will cause the entire network to reconverge and cause links to change states ...

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