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Spanning tree vlan priority - Cisco 3550

Posted on 2010-09-16
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Last Modified: 2012-05-10
Hello EE. I have a question about Sopanning-tree vlan priority. We have 2 x  Cisco 3550 core switches.for redunancy. Vlans has always been a bit of a grey area for me. I was looking at the switch configurations and noticed there was a slight difference between them:
 
Core 1
spanning-tree mode pvst
spanning-tree extend system-id
spanning-tree vlan 1-3,5,8,11,14,20-21,24,100 priority 12288
spanning-tree vlan 4,31,101-102 priority 24576
!
vlan internal allocation policy ascending

--------------------------------------------------------------

Core 2
spanning-tree mode pvst
spanning-tree extend system-id
spanning-tree vlan 1-6,8,11,14,20-21,24,100 priority 16384
!
vlan internal allocation policy ascending

Should these be the same?  Vlan 101 - 102 is missing altogether from Core 2.
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Question by:InSearchOf
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by:ragnarok89
ragnarok89 earned 200 total points
ID: 33696410
They don't have to be the same if the missing vlans don't even exist.

If they do exist, I would include them in your 2nd config.
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by:atlas_shuddered
ID: 33704385
Additionally, vlan 101-102, if added at future would be come up with the default priority of 32768 on Core 2.  In order to ensure that your vlans are propogated accurately across switches, you may also want to consider initiating a VTP domain.
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by:InSearchOf
ID: 33718847
Yes, the vlans exist so I would have to add them. Does it matter if all the vlans have the same priority as is the case it the Core 2 configuration?

Does VTP add any extra traffic to a switch?
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atlas_shuddered earned 300 total points
ID: 33719668
VTP adds a minimal amount of traffic.  As long as you aren't trying to run across a 10mb link, it isn't enough to concern over.
The priority will only need to be set on the the switch you desire to run as your root bridge (for STP).  This setting simply ensures that the desired switch is elected root and thus becomes the point by which all traffic must pass.  On your secondary, set the priority to -Core 1 default priority (12288 plus 1 and 16284 plus 1 respective to your vlan priorities noted above).  This will enable your Core 1 to come up as the root bridge until failover, at which point Core 2 would subsume the role.  A note on this, once you make the changes, be sure to validate that Core 1 comes up as the root (show root).  If it doesn't, tweak as below.
If you are setting up vlans on in a segmented fashion, meaning they aren't to propogated across all switches or you identify a different switch which will work better as your STP root switch, then configure your priority there.  As far as the priority being the same on all vlans, it is helpful, but not necessary, so long as the priority you set to the vlan leads to the desired root bridge election and failover sequence.
A general rule of thumb is to set the priority for the root, in each vlan, to 8194.  Another option, if the root is never going to change, ever, ever, ever, then you can set it to 1 but this is not recommended.
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by:InSearchOf
ID: 33742689
Thanks for the info.
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