Hey everyone!
So I feel like I have a good understanding of what to expect from VPC and STP, but now the lab is performing in an unexpected way (possibly because I 'don't' have a good understanding after all?). Here is my topology (all links are 10G, C&D extend one VPC to 7KA and one to 7KB):
Now, I understand that the VPC domain uses the Bridge ID of the VPC Primary (in this case, 7KC). So the logical domain should like this (basically a typical STP triangle):
Note that the VPC domain (should) lose the tie-breaker with both 7KA and 7KB. As such, I would expect the VPCs to be in a blocking state for each respective vlan. However, when I check my blocked ports, 7KA and 7KB are blocking:
N7K-A(config)# sh spann block
Name Blocked Interfaces List
-------------------- ------------------------------------
VLAN0020 Po100
Number of blocked ports (segments) in the system : 1
N7K-A(config)# sh spann root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
---------------- -------------------- ------- ----- --- --- ----------------
VLAN0010 4106 e4c7.2208.c4c2 0 2 20 15 This bridge is root
VLAN0020 4116 e4c7.2208.c4c3 1 2 20 15 port-channel1337
I ran two follow-up tests:
- Remove 7KA from the topology. If for any reason the VPC domain/primary is winning the tie-breaker with 7KB, it should become the root. However, 7KB becomes the root bridge (winning the tie-breaker over the VPC/primary).
- Change 7KB to have 'backup priority' of 8192 for vlan 10. This fixed the issue - the blocked port moved to the VPC.
N7K-C# sh spann block
Name Blocked Interfaces List
-------------------- ------------------------------------
VLAN0010 Po200
VLAN0020 Po100
Is this a function of VPC? Any other thoughts or ideas?? Thanks in advance :D
Jeff