Cisco ACI vPC Port-Channel Numbering?

[Robin Martinez]

It appears, at least with our configuration, that the port-channel numbers across the leaf switches in a vPC pair tend to not match.

For instance, a given vPC might be Port-channel7 on Leaf1 while it's Port-channel3 on Leaf2. Sometimes they do align but more often it seems they don't.

I understand that the numbering is really only locally significant, so it doesn't really matter for forwarding but it does make monitoring and troubleshooting a bit complicated when using the CLI or SNMP.

Is there any way to make sure these always align? Or are they just randomly generated every time they're configured/rebooted? There was nothing obvious that I could find.

[RedNectar]

Hi @Robin Martinez ,

Someone who has gone down this rabbit hole before may have a better answer, but my belief is (after 10yrs of configuring them), the PortChannel numbers are allocated at the time of creation and will stay the same through reboots/upgrades etc. Delete the VPC and re-create it it is likely to get a different PC number.

However, I've never documented or tested this, so I can't say I'm 100% sure.

[julian.bendix]

As far as I have seen so for, you are absolutely right @RedNectar !

I have put many VPC Port-Channels into consideration of monitoring systems before.. and the number does not change on reboots.
However is chosen for you upon creation and cannot be chosen manually.

BR Jules

[Robin Martinez]

Have you seen the scenario where the port-channel number is not matched between the vPC switch pair?

For instance, I'm looking at one right now where the vPC ID matches (of course) but it's Port-channel2 on Leaf1 and it's Port-channel3 on Leaf2 which seems odd. Everything works from a data plane perspective just as you'd expect.

[RedNectar]

Hi @Robin Martinez ,

Have you seen the scenario where the port-channel number is not matched between the vPC switch pair?

You bet. About 90% of the time the port channel numbers are different on each leaf.

But here's a tip if you want to find the VPCs in the Inventory, navigate to Fabric > Inventory >> Pod1 > Leaf-nnnn > Interfaces > PC Interface and you'll find your VPC. This is MUCH easier than Fabric > Inventory >> Pod1 > Leaf-nnnn > Interfaces > VPC Interfaces > nn > > Holy_sh_t_which_number_do_I_pick?  and then finding your VPC after eventually guessing the right number.

Another tip is to follow my naming conventions to find the relevant VPC easily by name. E.g. a VPC named L1201..1202:1:35_VPCIPG defines a VPC on interface 1/35 on Leaves 1201 and 1202

For instance, I'm looking at one right now where the vPC ID matches (of course) but it's Port-channel2 on Leaf1 and it's Port-channel3 on Leaf2 which seems odd. Everything works from a data plane perspective just as you'd expect.

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That's perfectly normal. Take 10 deep breaths and learn to live with it!

[julian.bendix]

Yes, all the time, that is completely fine, just as @RedNectar already explained.

[Robin Martinez]

The mismatch between port channel numbers is certainly annoying but we can easily deal with it.

The more curious part to me is the fact that they change after a reboot in our environment but seemingly not in others. I did go back and verify that after we rebooted a leaf last month for maintenance, all of the port channel numbers rearranged. Po6 became Po2, Po1 was suddenly Po4, etc. The vPC IDs do remain consistent.

I guess that will need to be a TAC case.

[RedNectar]

@Robin Martinez ,

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I did go back and verify that after we rebooted a leaf last month for maintenance, all of the port channel numbers rearranged. Po6 became Po2, Po1 was suddenly Po4, etc. The vPC IDs do remain consistent.

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Thanks for this gem. I'm going to play around a bit more and see if I can make it happen (when I get a spare day to play)