Do you know your Spanning-tree?

I have customers tell me all the time that spanning-tree is broken or it is worthless and they want to cut it off. The problem is not spanning-tree, it is the fact you do not know it. Most people open a switch box, install a few lines of code (VLANs, IP Addresses, username, and password) and send it on its way. Since 802.1d spanning-tree is on by default, they never look. This is where the problem begins. By default, the priority is 32768, so the lowest MAC address wins the root bridge election. You say, “I only have one path in the network so root bridge does not matter.” Well that is mostly true. First, do you want the oldest switch in your network making topology change decisions for a protocol that takes 50 seconds to converge? Second, do you want to run your network or let it run you?

As a Network Administrator, you need to know your network or you cannot effectively maintain it. You have to dictate what is going on in the network. It is up to you to stop the Gremlins in the network by setting up the network to fit your organization.

Configure the root bridge priority on the core switch at the top of the Layer 2 network with a lower priority than 32768. This will make it the root bridge for the spanning-tree topology. I use 8192 as my priority to keep it lower but still allow room for growth in the network. You can use 0, but that is the lowest you can go and it leaves no room to grow.

Configure rapid spanning-tree on your network. Rapid spanning-tree is based off of the 8.0.1w standard and converges faster than 802.1d spanning-tree (around 5 seconds). This is a suggestion and not a rule. You have to know your network before you make this change. Even though it is a standard, not all vendors play nice together. I have tested, Ruckus/Brocade, Cisco, HP, Adtran, and Nortel/Avaya for interoperability. All, but Avaya/Nortel play well together with a bit of tweaking. Avaya/Nortel needs to be in a single instance configuration to be compatible. You can look on the NetAdmin site for interoperability documents.

Lastly, just run the show spanning-tree command (or something close) to see how your spanning-tree is configured. Those show commands, a piece of paper and pencil, and a few minutes of your time will make your network stronger and you a more capable admin. My goal is to help every admin become the “Maytag Man” (if you are too young to know the reference, Google it). I want us all to be drinking coffee and letting everyone thing we are magicians behind the curtain as the whole thing runs like a well-oiled machine.

David Ellis
Systems Engineer
Ruckus Networks/Brocade

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