Cisco Nexus vPC Failover Scenarios and Troubleshooting Checklist

A vPC lets two Nexus switches appear as a single port channel to a downstream device, giving Layer 2 multipathing — but the failover behavior depends entirely on which link fails and in what order. Understanding the failure reactions is what separates a well-run vPC domain from one that melts down during maintenance. This article explains the three canonical vPC failure scenarios and provides the troubleshooting checklist used to verify a healthy domain, based on the Cisco Community knowledge-base article.

Failure Scenario 1: Peer-Link Failure

When the vPC peer-link fails, each peer checks the remote peer's status over the peer-keepalive (pk-link) heartbeat — 1–2 packets per second at ~64 bytes; failover triggers after 3 missed heartbeats. If both peers are alive, the secondary switch disables all its vPC ports to prevent loops. Data keeps forwarding down the remaining active port-channel ports, with failover gated on CFS message failure or UDLD/link-state detection.

Failure Scenario 2: Peer-Link and Keepalive Both Fail

This double failure is the hardest to discover: HSRP/PIM/STP fail to become active on the secondary, data continues forwarding but learning breaks for new flows. Using the management port as the keepalive link at least alerts the NOC via loss of management access — a valuable side effect.

Failure Scenario 3: One Peer Switch Fails

The remaining switch keeps forwarding data for the vPC — the cleanest failure mode and the one the design is built for.

Type-1 and Type-2 Consistency Checks

Before bringing up a vPC, the two peers exchange configuration to verify compatibility. Type-1 parameters (e.g. VLANs, STP mode, LACP mode) must match exactly — a mismatch drops the vPC on the secondary (graceful consistency check default). Type-2 parameters are less critical and do not bring the vPC down.

Troubleshooting Checklist

switch# show vpc
switch# show vpc brief
switch# show vpc consistency-parameters

• Is the peer-keepalive link in a separate VRF (or a management switch on the mgmt ports)?
• Are the keepalive source/destination IPs reachable from that VRF? (ping vrf)
• Is the peer-link a Layer 2 port-channel trunk carrying only vPC VLANs?
• Is the vPC number identical on both peers for each downstream port-channel?
• Are Type-1 parameters identical? Check with show vpc consistency-parameters.

vPC is the Nexus answer to the same problem MLAG solves on Arista — see Arista EOS MLAG 详解 for the comparison, and Cisco EVPN/VXLAN Multisite 排障 for EVPN integration context.

原文链接：https://community.cisco.com/t5/data-center-and-cloud-knowledge-base/vpc-failover-scenarios-and-troubleshooting-checklist/ta-p/3143104