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New Member

change of spanning-tree root


Would appreciate some advise on the following:

The network has already been configured with spanning-tree root primary as well as secondary.

Reassigning another switch to be the spanning-tree root primary/secondary, will it cause a downtime in the network? If yes, how long?




Re: change of spanning-tree root

If you are assigning a root primary to a switch that is currently not a primary, that means there will be STP re-convergence for 45 secs (15 secs for each stage listening, learning, blocking/forwarding), and for the whole network to converge, depends on how big the is the network. After root priority is adjusted it immediately sends out BPDU, the downstream switches gets this BPDU and process it immediately, if the new BPDU does not beat the root switch it already knows, it ignore that BPDU and nothing happes with the STP, if the new BPDU however, have lower prioirty,let's say, then immediately it goes to the stages (45 secs), this is for every switch. so, this depends on how long it took the end of that tree to get the new BPDU and add 45 secs to that. On a perfect scenario this should not take no more than 5 mins. But in Murphy's world depends what kind of problem you run into. This will be the network interrruption you are asking about. If you are assigning primary root role to a switch that is already a root then I don't anticipate any convergence but whenever STP is tweaked, it is always a good idea to schedule a downtime, you really never know what could happen.

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New Member

Re: change of spanning-tree root

Thanks, bosalaza.

New Member

Re: change of spanning-tree root


Which version of Spanning Tree are you currently using as PVST and MST have a much lower convergence time?

Re: change of spanning-tree root

I'm assuming PVST (not rapid-PVST nor MST, that should behave better)

It is very hard to give an exact estimate of a downtime. First, it's not going to be a global downtime. You can basically compare the topology with your current root bridge and the one with your new root bridge. The ports that need to block in the new topology will block quickly, in a matter of few seconds. However, the ports that were blocked in the old topology and that need to be forwarding in the new topology will take a little bit more than 30 seconds to become forwarding (15 second listening + 15 second learning phases). The topology change mechanism will age out stale CAM entries in 15 seconds. If you add a little margin for BPDU propagation, I would estimate that you are looking at a connectivity loss of about a minute, in part of your network.

The more blocked ports need to move from the old to the new topology, the most connectivity loss you will experience. Some feature like uplinfast are able to switchover quickly between their uplinks in case of root ID changing and will reduce the downtime dramatically. It's mainly the core bridges that will take time to unblock their port.

Note that you can expect better convergence time when a better root is introduced in the network (you are lowering the numerical value of the secondary root priority so that it takes over the primary) than when the primary root is downgraded into secondary (you increase the numerical value of the primary root so that it becomes worse than the secondary).