N6K vPC peer-switch changing STP priority

[whistleblower14]

hi all,

I‘ve a question regarding a vPC pair consisting of 2x N6K switches which act configured with vpc peer-switch command currently as STP root!

I need to make another device in the network the new STP root and so the priority value of 0 has to be changed… can somebody give me an advise how this should be done? I mean, do I‘ve to disable the peer-switch function and change the priority values on both switches to different one and after that enable peer-switch again? or is it suggested to change only the values on the first and than on the second N6K?

thanks for every feedback in advance!

[Sergiu.Daniluk]

Hi @whistleblower14 

Usually, when you have vPC peer-switch enabled on your nexus switches, it's recommended that those switches to be the STP root of the network. If you plan on changing the root to a different switch, then you can simply remove the peer-switch command, and supposing you have priority 0 on the two vpc peers, then change it to a bigger value. Note that this actions will generate an impact in stp domain.

Also, another recommendation, avoid using 0 as stp priority on any switch. 

Take care,

Sergiu

[whistleblower14]

hi @Sergiu.Daniluk 

thank you very much for your response on my question!

so disabling the "peer-switch" configuration under the vPC domain on both member switches and after that chaning the STP priority to default 32768 should work!

I´ve always thought to enable this "peer-switch" configuration on all VPC-Paired switches but if I understand you correctly this should only be the case on the Root vPC Switch-pair?

[Sergiu.Daniluk]

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thank you very much for your response on my question!

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Your very welcome!

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so disabling the "peer-switch" configuration under the vPC domain on both member switches and after that chaning the STP priority to default 32768 should work!

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Yes. The order is not that relevant, because impact of STP reconvergence will be there anyway. 

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I´ve always thought to enable this "peer-switch" configuration on all VPC-Paired switches but if I understand you correctly this should only be the case on the Root vPC Switch-pair?

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Correct. This is from config guide:

The vPC peer switch feature was added to Cisco NX-OS to address performance concerns around STP convergence. This feature allows a pair of Cisco Nexus 9000 Series devices to appear as a single STP root in the Layer 2 topology. This feature eliminates the need to pin the STP root to the vPC primary switch and improves vPC convergence if the vPC primary switch fails.

Reference: https://www.cisco.com/c/en/us/td/docs/dcn/nx-os/nexus9000/101x/configuration/interfaces/cisco-nexus-9000-nx-os-interfaces-configuration-guide-101x/b-cisco-nexus-9000-nx-os-interfaces-configuration-guide-93x_chapter_01000.html#concept_5437862401DC4435...

Take care,

Sergiu

[whistleblower14]

OK - but only assumed - please see the sketch in the attachment!

Both connections or both switches in vPC domain 11 fail and the STP root changes, for example to the switches in vpc domain 10... wouldn't it also make sense in such case if the"peer switch" configuration were activated on these switches too?

[Sergiu.Daniluk]

Depends. I see you have some core devices, switches I suppose. Maybe those ones would be better preferred as STP root. But if you really want vpc domain 10 to be your backup, then I do not see a problem not having "peer-switch" configured, since anyway it will be a timed time when vpc domain 10 will be root.

Take care,

Sergiu

[jairhern]

Just Increase STP Priority at N6Ks, however first check the command Show spanning-tree root in both N6Ks, after increase the STP priority some networks intermittencies can occurs, for example after change the STP priority run the command show spannig-tree blockedports in both N6Ks and in the New STP root

reagards