spantree macreduction on production swtiches

Unanswered Question
Aug 10th, 2007

Does anyone know what the impact of enabling this feature on a production switch. We would like to enable extended VLANs on our switches but were concerned about the impact. I would appreciate any comments especially for those that have done this before in a production environment. Thanks.

I have this problem too.
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m-mueller Sun, 08/12/2007 - 22:44

Hello orlanmag,

we have done this before on many Cat6509 Switches with CAT-OS or IOS on it. There wasn't any impact. The production was still going on ;-)


Francois Tallet Mon, 08/13/2007 - 09:28

The main problem is that the feature will change the priority of your switches. Nothing catastrophic, but you have to be careful, especially for the switches that you have configured for a particular priority.



orlanmarq Tue, 08/14/2007 - 11:15


Thanks for the reply.

So, all VLAN priorities previosly manually configured will change to the default priority. Is this right? Thanks.

Francois Tallet Tue, 08/14/2007 - 13:28

Well, I don't know for CatOS, but IOS is trying to re-generate a new priority from the previously configured priority. Without extended sysid, the priority is encoded on 16 bits. When you enable the feature, the 4 highest priority bits of your configured priority are kept, the 12 lower priority bits are overwritten with the instance ID.

Note that it will affect also the instance that have no priority configured. For example, with no configuration, your priority is 32768. Now with extended sysid it will be, 32769 for vlan 1, 32770 for vlan 2 and so on.

My point to focus on the bridges where you have configured a priority was because I had in mind the root and secondary root bridges, the most important configuration in your network. A change in the root bridge ID could affect the whole topology of your network. A change in a "normal" non-root bridge ID could also have an impact, but it will only be local to this bridge (a blocked port could move from one end of a link to the other for instance) and are much less important.

As a example, consider that for all the vlans your root bridge had a priority of 9000 and your secondary root of 10000 (weird figures, but that's an example;-).

Configure extended sysid on the secondary root bridge and its priority will become 8192 + vlan ID. It will suddenly become root for the first 9000-8192 = 808 vlans (well, actually there will be a tie based on the mac address for vlan 808;-). That's this kind of stuff you have to be aware of while migrating.




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