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I'm new to this SE forum so I hope I don't sound too dumb.

I was setting up a new rack with servers and switches and I connected the switches up to the rest of the network but I forgot to hook up the vpc peer link bewteen two of my switches on the rack (the network guy said the two switches basically act as one looking at it from a level 2 perspective, the switches are an irf pair I guess).

He went on to say by not connecting the two vpc peer links it created a loop in the network, which is why I had trouble communicating with some of the servers in the rack.

How does not having the vpc peer links create a loop? and what exactly is going on that makes it a loop?

I doubt it matters, but I'm using cisco Nexus 3548 switches.

Thanks!

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    WTF is a "vpn peer link"? That's not a Cisco term. – Ricky Beam Jun 15 '16 at 18:17
  • We could answer your question if you provided a simple diagram and the switch configurations. Otherwise, we're not sure what you are asking. – Ron Trunk Jun 15 '16 at 20:42
  • yeah I think I do to I will correct that. – scarlso9 Jun 15 '16 at 20:44
  • Odd, because 3548's don't have VSS capability. – Ron Trunk Jun 15 '16 at 20:45
  • @RonTrunk this is not VSS, just VPC (layer 2 only, just to establish port channels that are split between two parent Nexus devices for redundancy).The VPC peer link allows the two VPC peers (your Nexus 3ks) to exchange the information necessary to make VPC work. So if your peer links are down, you end up with a really wierd split-brain situation where your port-channel is split between two devices that can't see each other. The documentation explicitely states something along the lines of "whatever you do, make sure the peer link can never be completely down, or else". – Jeremy Gibbons Jun 15 '16 at 20:55
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You probably didn't create a loop in your network. However, without the peer links, your two switches, instead of acting as one switch, are acting as two independent switches and dropping half of your traffic. That's likely why you couldn't communicate with some of your servers.

A short explanation:

Your vPC sends traffic across both links to each of the switches. With the vPC peer links, the switches can both forward the frames to the destination.
Without the link, one of the switches will shut down the links to prevent loops. So half your traffic is not reaching the destination.

One of my former colleagues wrote a more detailed explanation here.

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