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Extending VPC vlan's across single connected (into 7K) L2 path

I've been asked to comment on this design as a temporary solution.

Nexus infrastructure is pair of N7K switches, with dual homed 5K/2K devices connected, links use vPC. They want to extend vlan's from the 5K/2K side across a L2 path into another DC, this path is only connected into one of the N7K switches, call this 7K switch A.

I'm concerned that packets from the 5K/2K side back to the remote DC could get dropped due to the vPC loop dectection process. If a packet from the 5K/2K side traverses the path into the same N7K (7K switch A) as the L2 extension, I dont see a problem.

But, if the etherchannel load balance in the 5K sends traffic to the packet to N7K B, in order to get to the L2 path extension to the remote DC the packet gets forwarded across the vPC.

My understanding is that this packet will get dropped by N7K A as its local L2 path to the remote DC is up, is this the case?

I see a solution is to plug the L2 path into a 2K, but before I suggest this is my understanding of the loop detection process correct?



Re: Extending VPC vlan's across single connected (into 7K) L2 pa


Packets going northbound in your diagram wouldn't be affected by the vPC loop-advoidance mechanism, which only prevents packets received from the peer-link being transmitted out of a vPC where the peer also has an active member. As your link is not a vPC it's allowed for packets to take the suboptimal path.

Please be aware that it's not officially supported to connect anything than an end host to a fabric extender host interface, documented as "Do not connect a bridge or switch to a host interface. These interfaces are designed to provide end host or server connectivity." [1]

Hope this helps,


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