VPC Uplink Fail Scenario

[NAGISWAREN2]

Hi All,

I have VPC pairs with L3 Uplink with ISIS on both nexus. How does the VPC pair forward traffic if say Primary Nexus Uplink fails (whether physical uplink port down or ISIS routing down) or vice versa ? There is no ISIS between the pairs. 

[chrihussey]

So if each switch in the VPC pair has its own L3 uplink and there is no routing occurring between the two VPC peer switches, I would expect traffic would be dropped as a result. If one of the peer switches doesn't have the routes it will not by default just send it to its peer switch.

[NAGISWAREN2]

Thanks. But does that mean , if I had Layer 2 Uplink on both VPC peer switch, it will forward traffic via VPC peer link in the case of Uplink is down ?

[Manoj Papisetty]

Yes, it would. Essentially it depends on the destination mac address of the packet entering a vpc peer and the next hop it has to take. If you have L3 uplinks that go down, you would have a problem for sure. Since layer 3 routing over vpc has been changing with versions and a lot of improvements have kicked in, it would be helpful if you have a specific scenario and a question.

[NAGISWAREN2]

Thanks Manoj. My setup as below

Two Nexus setup as VPC pairs. Each Nexus has L3 Uplink to C9500 Catalyst using ISIS Routing. But no ISIS between the nexus. The VPC pairs also run HSRP towards the LAN Servers(Acting as Gateway for Servers) . I understand how nexus behave when Peer Link / Keep alive link or Peer switch down. So now question is how does the nexus behave if the L3 Uplink fails, whether the Uplink interface down or ISIS peeing down. ? And What can be done to ensure traffic routing even if the L3 Uplink fails.

[chrihussey]

So if you have L3 uplinks on the vPC pair, the only way that I know of to keep routing working would be to L3 peer between the two vPC switches. That is the way I have always done it.

I have never done the L2 uplinks, however if you mean a common VLAN between the two vPCs peering to the uplink switches/routers, it is possible. However there may be a catch in that the VLAN being used for this purpose may require its own dedicated physical link between the vPC switches. That is because as a general rule, any VLAN that is not participating in the vPC, should not traverse the peer link. There may be issues. That's the case anyway with the 5K nexus. If in fact you are establishing uplinks with vPCs, then it won't be an issue.

Hope that makes sense and helps.