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Back to back N7k and N5K

We have a client where the current setup is a VPC pair of Nexus 7Ks in the core.  As well as a Nexus VPC pair of N5Ks in the DC.  The N5Ks and the N7Ks are currently back-to-back connected with VPCs.  Now, the client is looking to deploy NSX in the DC on UCS.  Because of the L3 concerns of running OSPF across VPC connections, we are needing to remove VPCs between the UCS and the Core 7Ks.  Can we remove the VPCs and rely on plain ol spanning-tree and be OK?  The N5Ks and N7Ks would still be cross-connected to each other. 

Philip D'Ath

You will still can use VPC links  between the Nexus switches.  I don't see any issues with dropping VPC to the UCS platform, because UCS does not forward packets at layer 2 like a switch, so you should not be able to form a loop.

No, I don't think I can.  There are no VPCs from the 5K to the FIs.  But there are VPCs between the 7K and 5K.  That would have us establishing an OSPF neighbor relationship between the N7Ks and the NSX distributed routers across VPC links.  That's a no no.

If the UCS is going to peer with the 7ks over the VPC vlan that that is not supported. Your design is similar to what is shown in the vpc best practice document, page 82, fig 61. It shows what is not supported from a L3 peering point of view.


Yes, exactly.  So I'm trying to figure out what I have to do to correct this setup.  Can I just remove the VPCs and rely on spanning tree for the redundant links?

I can think of couple of options  to ensure that the UCS can peer with the 7Ks.

1. Remove the vpc config between the 5k and 7k and make it run in a traditional non VPC STP setup. Ofcourse this is drastic change and may seem an overkill for making one thing work. Also you end up losing a lot of bandwidth between the tiers due to stp blocking links if you follow this step. From my perspective this is a theoretical option just for discussion purpose.

2. A more feasible option would be to move the UCS to the 7K and connect it via a traditional way using non vpc vlan

Maybe others on the forum can chime in on other options as well.


I think my only option is Number 1.  Leave the VPC peering for other connections off of the N7K and N5Ks.  Remove VPCs between the 5Ks and the 7Ks, yes, with less bandwidth.  The UCS cannot connect directly to the 7K as they are physically separated in different parts of the building.  Would I need an additional trunk link between the 7Ks beyond the current VPC peer link?