Could anyone please advise of a reason not to set the Designated Root Priority of the root bridge to 0. I normally configure primary root as 100 and secondary as 200. A colleague always sets primary to 0 and secondary to the default 32768. I need a good reason.!!
The terminology you use has confused me. When you say 'Designated Root Priority' I assume you mean 'bridge priority' which is used to determine the root bridge. Setting the bridge priority of 0 will have the same effect as setting it to 100 - as long as all the other switches are set to the default of 32,768. The only problem if use set the priority to 0, is if you add another switch to the network which you want to take over as the root bridge. You could set the bridge priority 0 (which is the same as the current root) and the tie-breaker would be the switch with the lowest MAC address becomes the root. By using 100 as the priority, if you wanted to add a switch as the root you would simply set a priority less than 100 - ie 50.
The main problem with your colleagues setup is the backup root bridge is set to 32,768 whish is the default. Therefore, if the root bridge fails, all the other switches have the default priority of 32768. In this case, as I said earlier, the lowest MAC would assume the role of root bridge. This may not be the switch he had in mind - I would suggest he lower the priority of the secondary switch to for example 16,000 to prevent going into the lowest MAC tie-breaker.
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