The Hot Standby Router Protocol (HSRP) provides redundancy by allowing multiple routers on a single LAN to share a virtual IP and MAC address. This virtual IP address is configured as the default gateway on the hosts. From the group of routers configured in a HSRP group, one router is elected as the active router and another as a standby router. The active router assumes the role of forwarding packets sent to the virtual IP address. Devices that are running HSRP send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers.
The active router sources hello packets from its configured IP address and the HSRP virtual MAC address while the standby router sources hellos from its configured IP address and the Burned-In MAC Address (BIA). This use of source addressing is necessary so HSRP routers can correctly identify each other. Multiple standby groups may co-exist and overlap on a LAN, and individual routers may participate in multiple groups.
There are two reasons why the STANDBY-3-DIFFVIP1 error message occurs:
The virtual IP address in the hello packets sent by the active router does not match with the virtual IP address configured on the standby routers for the same group. The root cause for this error is the virtual IP address misconfiguration on any of the HSRP routers.
There is inter-VLAN leakage due to bridging loops in the switch.
The solution is different depending on the cause of the error message.
If you suspect virtual IP address misconfiguration on any of the HSRP routers, check all HSRP routers in the same group for the virtual IP address configured. To verify the hello packets for virtual IP address mismatch, issue the debug standby command.
If you suspect inter-VLAN leakage due to bridging loops in the switch, perform these steps to resolve the error:
Identify the path that packets should take between end-nodes.
If there is a router on this path:
Troubleshoot the path from first switch to Router
Then troubleshoot from Router to second switch.
Connect to each switch on the path and check the status of the ports being used on the path between end-nodes.