Cisco Nexus - vPC Peer-Switch

The vPC Peer-Switch feature allows a pair of vPC peer devices to appear as a single logical Spanning Tree Protocol root.

With vPC Peer-Switch enabled downstream devices see both switches as the STP root.

Without vPC Peer-Switch enabled downstream devices still have to funnel Layer 2 traffic via the STP root using the traditional STP traffic flow.

The main advantage of vPC peer-swtich 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 can create around 3 seconds of traffic disruption 
• With vPC peer-switch
• traffic disruption is lowered to sub-second value 
• This is since peer device down an up events do not generate any Rapid Spanning Tree Protocol Sync behavior, because, there is no logical change in the L2 topology.

Things to keep in mind:

• It is a Cisco recommended best practice to activate vPC peer-switch within a vPC environment.
• vPC peer-switch must be configured on both vPC peer devices
• When vPC peer-swtich is activated, both vPC peer devices MUST have the same spanning tree configuration 
• This means the Spanning Tree Protocol priority must match for all vPC VLANs

Configuration Example

vPC Peer Device #1

NEXUS-SW01# conf t
NEXUS-SW01(conf)# spanning-tree vlan 1,10,20,100 priority 12288
NEXUS-SW01(conf)# vpc domain 1
NEXUS-SW01(conf-vpc-domain)# peer-switch
NEXUS-SW01(conf-vpc-domain)# end
NEXUS-SW01# copy running-config startup-config

NEXUS-SW02# conf t
NEXUS-SW02(conf)# spanning-tree vlan 1,10,20,100 priority 12288
NEXUS-SW02(conf)# vpc domain 1
NEXUS-SW02(conf-vpc-domain)# peer-switch
NEXUS-SW02(conf-vpc-domain)# end
NEXUS-SW02# copy running-config startup-config