Hello Scott_22,
In a vPC domain the device in charge of processing and sending BPDUs is the vPC primary, even if the root is the secondary switch, what happens in this scenario is that when the secondary switch in the vPC (the root), receives a BPDU, it proxies that BPDU to the primary switch across the peer-link so it can be processed. The best practice is configure the vPC primary switch as the root.
This is from the control plane perspective, now, from data plane perspective, both of the switches will be able to receive and forward regular traffic since in a vPC environment we won't have STP blocked ports, loops are avoided using the vPC loop avoidance mechanism of the vPC.
There is a feature that was implemented to enhance the STP operation in a vPC domain, it is called peer-switch. The main advantage of vPC peer-switch is the improvement in term of convergence time during vPC primary peer device failure/recovery. Without vPC peer-switch feature, vPC primary peer device failure and recovery usually create around 3 seconds of traffic disruption (for south to north traffic). With vPC peer-switch, traffic disruption is lowered to sub-second value because peer device down an up events do not generate any Rapid Spanning Tree Protocol Sync behavior (from a STP standpoint, there is no change in L2 topology).
When peer-switch is enabled, each Nexus switch shares a virtual bridge ID which allows both switches to act as root for the VLAN, so both switches are capable to process BPDUs.
Note: For proper behavior, VLAN priorities on both Nexus 7000 switches should be configured the same.
Below some documentation about this:
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/interfaces/521_N11/b_5k_Interfaces_Config_Guide_Release_521N11/b_5k_Interfaces_Config_Guide_Release_521N11_chapter_0101.html
https://www.cisco.com/c/dam/en/us/td/docs/switches/datacenter/sw/design/vpc_design/vpc_best_practices_design_guide.pdf
HTH
- Orlando -
- Orlando -
