Need advice re spanning tree on a mixed vPC/non-vPC deployment

[d-fillmore]

Hi - I need to run a pair of Nexus 5ks running vPCs northbound to a pair of UCS chassis for VLAN X.

I also need to connect each 5K southbound to a single Cat6K - 5K1 to 6K1, 5K2 to 6K2.

I realise these will be orphan ports and this is a bad design but this is only a temporary measure until we can get some additional patching done. 

I'm trying to understand how the spanning tree is going to affect the VPC peer link, as both the 6Ks are connected together and are the root and secondary root for VLAN X. Based on this, I'd expect the VPC peer link to be blocking on one of the 5Ks for VLAN X, but how will this affect the north facing VPCs to the UCSs?

Any advice would be appreciated

Dom

[Manuel Velasco]

Can you provide a paint diagram of how this topology looks like? Are you saying that between the N5Ks and the Cat6Ks, there is not a VPC in between?

[d-fillmore]

Hi Manuel - Yeah that's correct. 5K1 is connected to 6K1 and then 5K2 is connected to 6K2.

There is no direct connectivity between 5K1 and 6K2, and 5K2 and 6K1. 6K1 & 6K2 are connected to each other so on a diagram, it would look like a square for this connectivity

Does this clarify the topology or do you still want a diagram

[Manuel Velasco]

Assuming your topology is like the one attached to this post, one of the 6Ks will be set to blocking state (secondary root).  If traffic from fabric B needs goes to N5K_2 the traffic will go from the peer link and up to the Cat6K that is not blocking (root). 

Let me know if this make sense or if the diagram is not correct.

[d-fillmore]

Hi Manuel,

Yes your diagram is correct - I feel bad for not drawing it now!

the vPC peer link is 2 x 10Gb, the portchannels from the 5Ks to the 6Ks are 2 x 1Gb. Based on that, I would expect one of the 5Ks to be blocking on the link between them.

From what I've read, BA should mean that the vPC link is never blocking, but the part facing the 6Ks would probably mean the the vPC link is blocking, hence my confusion.

Another option is to make the 5Ks the root/secondary root, but I'm unsure how much impact there would be to traffic on that VLAN if the root bridge moved in a rpvst topology - probably just the time it would take to repopulate the mac address tables

Cheers, Dom