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I have been immersing myself in the Cisco STP documentation, but for the life of me I am not able to find anything relating to the toppology changes (aka ports logically going down) when certain commands are run.

Due being un-familar with STP when I set STP up, I missed some glaring best practices and I need to correct hem now. I am concern that when I do, the toppology is going to re-assess and the network will go down.

I need to enter the following on my core switches:

Sw1# spanning-tree vlan 2-400 priority 8192

Sw1# spanning-tree portfast bpduguard default

Sw1#(uplinks) spanning-tree loopguard default

Sw2# spanning-tree vlan 2-400 priority 16384

Sw2# spanning-tree portfast bpduguard default

Sw2#(uplinks) spanning-tree loopguard default

When entering the global commands will each switch recalculate the STP topology?

I assume that the interface command (spanning-tree loopguard root) will cause a logical shutdown like when entering spanning-tree portfast

Thanks for any feedback. Unfortunately I run a small shop and don't have the budget for test equipment.

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Without knowing if you have other switches and which switch is the root bridge I can't give you a specific answer.

with spanning-tree vlan 2-400 priority 8192 you lower the priority for Sw1 for vlan 2-400 which means it will become the root bridge unless there exists a switch with lower priority (lowest priority becomes root bridge).

So if Sw1 was not previously the root bridge, the switches will determine the shortest least-cost path to the root bridge which could mean that links changes state.

As for spanning-tree portfast bpduguard default this command enables bpduguard on all accessports with portfast enabled. If you have a switchport with portfast enabled connected to a client and it gets connected to a switch instead, the port will go into err-disable if a bpdu is received. It will stay in err-disable until you do a shutdown, no shutdown on the interface or if you have errdisable recovery configured.

spanning-tree loopguard default means that if a port which is supposed to receive bpdu's stops receiving them, the port will be disabled until it starts to receive bpdu's again. This to prevent loop because of Unidirectional link failures.

  • Thanks Jimmy, the problem I was having is that a switch across a WAN had become the root bridge for this VLAN and the link became unstable causing numerous TCNs. I assumed the priority command would make changes, but wanted to make sure. – Graham Aug 18 '15 at 16:03
  • @Graham Your welcome, but please mark the question as solved or clarify if you need more help so that we know if you want another answer or not. – Jimmy Aug 19 '15 at 5:24
  • I'm not sure why you run spanning-tree over a WAN link but if you receive BPDUs from the switch across the link you can see which priority it advertises and just set your own lower to become root bridge. With the command "show spanning-tree" you can see what priority the root bridge has under Root ID. – Jimmy Aug 19 '15 at 5:31

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