Nexus 9300 nx-os - VPC and MST Considerations

[TONY SMITH]

Hi,

I'm come up against this bug, or maybe it's not really a bug since there was apparently no intention to change the behaviour ...

"When a N9K VPC primary switch comes up after reload , the peer-link on Secondary VPC goes into ALT BLK state for the allowed vlans, if there are different STP MST priorities configured on peer switches."

https://bst.cloudapps.cisco.com/bugsearch/bug/CSCva10630

From reading the notes on that bug the recommended configuration if using both VPC and MST, is either to remove the "peer-switch" setting from the VPC domain, or to configure the same MST priority for both VPC peers.  I'm really wondering about the pros and cons of those two options and I don't think I'm really finding this in the configuration guides.  The guide for MST doesn't mention VPC at all.  And as far as I can see the guide for VPC doesn't really cover this detail either.

Any comments or suggestions?   For background the intention is for all VLANs to be included in the VPC domain, and all devices either dual homed by VPC to the Nexuses, or if they can't be dual homed they will connect to one of the access switches which themselves are VPC connected.

Thanks, Tony S

[Sergiu.Daniluk · Accepted solution]

Hi @TONY SMITH 

First rule in vPC Peer-Switch feature is to configure the same priority. That's simply because the two switches will own the same bridge ID (0023.04ee.beXX , where XX is the vpc domain number) and both will actively participate in STP (send BPDUs). If you have different priorities configured.. well that's not good :-). So the bug is not really a bug - is unsupported configuration, because of different priorities on peer-switch enabled switches.

Second rule for peer-switch - enable this feature only when your Nexus switches are root bridge in your STP domain.

Third rule - while configuring peer-switch w/ MST, do not map any vlan to MSTI 0. Always create a new instance and map the vlans to that instance.

If you can stick to this rules, you should be fine with peer-switch. Otherwise, better not use it.

Stay safe,

Sergiu

[TONY SMITH]

Thanks for quick summary.

[giorgio-lupi]

Hi,

my you kindly explain why the rule "do not map any vlan to MSTI 0"