Nexus vPC Peer-Switch - How is Virtual Bridge ID calculated?

[Jonathan Bekkers]

So... on reading through several documents and looking at live configurations, it appears as if when you enable vPC Peer-Switch on Nexus switches they will use a virtual Bridge ID for STP calculations.

From what I can see the virtual Bridge ID is always 0023.04ee.be01. This is great for my situation but I was wondering whether there is some sort of calculation used for this? What if we have multiple sets of vPC Nexus pairs on a Layer 2 broadcast domain and they both happen to have same priority and Bridge ID. Specifically I was thinking about a DC with vPC core and distribution layers on separate pairs of Nexus switches both running vPC and peer-switch. I honestly wouldn't keep STP root bridge on core and access the same in this topology but I was wondering what would happen if you did since they'd have the same virtual Bridge ID and STP root priority.

Has anyone come across this before? Is there some smarts within Nexus to generate a differing virtual Bridge ID in these conditions?

[Paul Chapman · Accepted solution]

Hi Jonathan -

The last byte of the vMAC is automatically generated from the vPC domain number.  In this case your vPC domain is 1.

It is generally recommended that each vPC domain have it's own unique number to prevent conflicts like this. (e.g. vpc 10, vpc 20, etc.)

The automatic vMAC can be manually overridden with the "system-mac" command under the vpc domain settings.  Please note that this value must be exactly the same on both members or you will have a Type 1 mismatch that will take your vPC secondary offline.  Converting from automatic to manual is disruptive and should only be done during initial setup or a maintenance window.

PSC

[Jonathan Bekkers]

Great answer. Thanks.