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New root bridge selection

We have a 35xx switch connected to our core 6500 that is the root for all VLANs in out network (selected through defaults). There is no physical loops in our network. The 6500 feeds multiple other closet switches.

If the 35xx were powered off would we lose connectivity to the other switches connected to the 6500 during the election of the new root?


Re: New root bridge selection


it's difficult to answer without your network topology knowedge.

But if I understand correctly:

All switches are having the default STP priority, 35xx switch is root because of lowest MAC address, there are no redundant connections in the network.

I suppose there is no other switch between 35xx and 6500, 3500 is connected only to 6500 and all other switches are cascaded with 6500 as the centre.

Under these conditions I would try to force 6500 to become the new root.

You can do it even before 35xx switching off. And because the path to the root from any other switch will not change, the network impact should be minimal.

I would choose the less important (or create a testing one) VLAN in your network and set STP priority for 6500 to become the root (the best way is to use "set spantree root ..." macro) for the testing VLAN.

When the 6500 becomes the root for testing VLAN you'll see the impact (should be none). You can change the root for other VLANs then and switch off the 35xx finally.

One general advice:

It's always better to set one chosen switch to become the root and another one as secondary root (using that macro, e.g.). You can choose the best one from topological point of view and you always know where the root is.




Re: New root bridge selection

I believe you'd lose connectivity for every VLAN that is participating in the new root election.

Your post is a little ambiguous; the 3500 is the root? or the 6500?

If the 3500 is the root for all VLANs, and it goes away, then every VLAN will go through the re-election process and no traffic will flow until the election is complete (depending on the size and topology, could be seconds to minutes).

If that is the case, then IMHO, you should change the priority for the bridges/switches such that the 6500 is the root for each VLAN. I'm assuming that you have the 6500 rigged for redundant power and, since it's the core of your network, set up to be the least likely to completely fail.

Make the 6500 the root, and setup the next most-protected switch to take over in case of component failure. When the failed component is replaced and announces itself, it should re-gain the root status automatically (but with a service interrution during election).

You can tweak some of the parameters to get your convergence time down, but be careful not to set them too low or you may cause some false failures.

Good Luck


Community Member

Re: New root bridge selection

Sorry, As I re-read my posting it is ambiguous. Yes, The 3500 is currently the root bridge for all VLANs. I will make my 6500 the root for all VLANs.

Thanks for the reply.

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