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HSRP standby IP address is reported as a duplicate IP address and the user receives the "STANDBY-3-DUPADDR" error message

Core Issue

These error messages do not necessarily indicate a Hot Standby Router Protocol (HSRP) problem. Rather, the error messages usually indicate a possible Spanning Tree Protocol (STP) loop or router-switch configuration issue.

The IP address in an HSRP message received on the interface is the same as the router's own IP address. This condition may be caused by a network loop, a misconfiguration, or a malfunctioning switch.

Resolution

As a workaround, configure an Access Control List (ACL) that prevents the active router from receiving its own multicast hello packet.

HSRP packets are sent with a User Datagram Protocol (UDP) source and a destination port of 1985, so the following  ACLs can be used to filter HSRP packets:

Hostname(config)#access-list 177 deny udp x.x.x.x any eq 1985
!-- x.x.x.x is the IP address of the active router.
Hostname(config)#access-list 177 permit ip any any 

Apply the ACL inbound on an interface as shown:

Hostname(config)#interface ethernet 0
Hostname(config-if)#ip access-group 177 in 

Refer to these documents for more information:

Other Errors, Warnings, and Log Messages

HSRP - "%STANDBY-3-DUPADDR"

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