[c-nsp] Nexus vPC loop avoidance details?

[Lincoln Dale]

On 23/04/2011, at 11:08 AM, Adrian Chung wrote:
> The 6500s each have two ten gigE interfaces in a port-channel connected up
> to vPCs on the 7K side.  On top of this, each 6500 is forming an OSPF
> adjacency with each 7K.  The adjacencies form without a problem, and we're
> not using peer-gateway.

running a L3 routing protocol over vPC member links between SVIs is not a supported configuration (as you state).

what you have may _seem_ to work today. that doesn't mean they will continue to work in future.

what may cause it to stop working?
well, for starters, OSPF uses a few different types of control packets.  some use link-local multicast, which if they arrive on the 'right' N7K will work.  but if they arrive on the 'wrong' N7K they will fail.

whether a device sends to the 'right' or 'wrong' N7K depends on which physical link it chooses to use in a LAG bundle.  as the neighboring device has no idea its a point-to-multipoint bundle, its not really in a position to choose the 'right' or 'wrong' link.


as to how vPC does loop avoidance, its sort of beside the point as to how it actually does it - just that it _does_ do it.
i don't think its a secret per-se as to how we do it, but what you've observed with routing protocols is somewhat orthogonal to that.


cheers,

lincoln.