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Community Member

6509 spantree priority

Hello all, first post.

I read in an earlier post that spantree priority only weighs in on a tie for STP root bridge determination.

My question is why would I set spantree priority on my 6509 for VLANs? (we have no redundant links anywhere) The integrator for the 6509 did this initially, but not for subsequent VLANs, I'm wondering if it's needed at all. If not, I can't find the command to remove the "set spantree priority 9 <vlanX>" statements in the config. ("clear spantree priority <vlanX>" resulted in errors.)

Thanks in advance

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Accepted Solutions
Cisco Employee

Re: 6509 spantree priority

Hi,

Unless there's a specific reason to do so (to force certain devices to be root, etc), it's generally best to leave the STP priorities to the default.

There's no command to remove "set spantree priority 9" per se, but to reset it to the default, simply configure the value from 9 back to the default setting.

HTH,

Bobby

*Please rate helpful posts.

3 REPLIES
Cisco Employee

Re: 6509 spantree priority

Hi,

Unless there's a specific reason to do so (to force certain devices to be root, etc), it's generally best to leave the STP priorities to the default.

There's no command to remove "set spantree priority 9" per se, but to reset it to the default, simply configure the value from 9 back to the default setting.

HTH,

Bobby

*Please rate helpful posts.

Community Member

Re: 6509 spantree priority

Perhaps the integrator intended the 6509 to be the 'authoritative' device for STP. The switches closer to the users (this 6509 is the core) should have lower priority (or no vote at all) on any matter concerning STP, VLAN assignment, VTP, etc.

Thanks for your prompt, helpful response.

-Kirt

Re: 6509 spantree priority

I think there is a slight confusion between the port priority, and the bridge priority.

It's the port priority that was discussed in earlier posts, and that is indeed not really very useful except in some very particular cases.

The bridge priority, allows you to specify where you want your root bridge. I would recommend you set at least one root bridge and one secondary root bridge (in case of failure of the latter). Your final topology will look like a star, centered on the root bridge. It's thus better to select a root bridge in a location where most of the traffic is going to (the bridge where the default gateway of your network is located, or where the most used servers are connected etc...).

Of course, if you have no redundancy at all in your network, this is not very important, because no link will be blocking. However, I would still recommend you identify the location where your root bridge should be and configure it explicitly. This way, if some redundant links are introduced later (by mistake or not;-)), there will be minimal impact on your network.

IMO, the two most important STP configs:

-1- setting the priority of the bridge that should be root (and secondary root).

-2- tuning the cost of the necessary ports so that your desired topology is enforced, if the default values are not ok (they are generally ok, because they are based on the cost).

Avoid tuning the timers if you are not confident in what you are doing (it's almost of no use with RSTP and MST anyway).

Regards,

Francois

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