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STP - setting the root bridge

Currently our root bridge is an access-layer switch because all switches are using the default priority and so the one with the lowest MAC address got elected.

So I'm going to set the root bridge to be our core 6509. Once I make the change, will it be transparent to the user because it is treated like a topology change notification, or will the whole network be down for 50 seconds while STP recalculates and configures?

I've only got twenty or so vlans (using PVST+) and I want the 6509 to be root for all active vlans. So I will use "set spantree root 1-1005" rather than specifying the individual vlans.

Any pitfalls?



Community Member

Re: STP - setting the root bridge

This root change most likely will not be transparent to the user as probably your topology would change so that a current forwarding link may have to block and vice versa. You should plan on a downtime for making this change. There is no harm in making the 6509 the root of all the vlans.

Community Member

Re: STP - setting the root bridge

I did exactly the same thing for a Cat5500 with about 20 VLANs and notcied no downtime at all. The thing with STP is that it is unpredictable and it could cause downtime so plan for it. I gues that your 6509 is the centre of your network, and that all Vlans originate from there? You wouldn't want to make the root bridge the 6509 for a VLAN that only ran from an access switch to an access switch. This would mean that certain data paths would have to make an extra hop through the 6509 when they wouldn't normally have to.

PS - it may be worth doing ne VLAN at a time. Start with a smallest VLAN (population wise) and see how that affects the network. After that, you can do 3 or 4 at a time.


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Re: STP - setting the root bridge


Your root bridge of your spanning tree should be in the center of the network if possible, the distribution layer is the typical choice. Spanning is usally transparent to end user, changes to the root bridge may cause loss of connection while spanning tree run it's algorithm to calculate the best loop free path within the network. Downtime should be considered.

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