HSRP Message Op Codes
Hot Standby Router Protocol (HSRP) is an FHRP that uses specific messages to communicate between the redundant gateways. The messages are defined using specific Op codes that appear in the Op Code field of the data portion of the UDP datagram used for communication.
These Op Codes are defined like so:
| Op Code | Name | Description |
|---|---|---|
| 0 | Hello | Sent periodically by active and standby routers to indicate they are operational. |
| 1 | Coup | Sent by a router that wants to take over as the active router. |
| 2 | Resign | Sent by the active router to relinquish its role, allowing the standby router to take over. |
| 3 | Advertise | Used in HSRP version 2 for additional message advertisement purposes. |
Op Codes 0 through 3 are defined in RFC 2281 which describes HSRP, while Op Code 3 is used in HSRPv2.
Links
https://networklessons.com/cisco/ccie-routing-switching/hsrp-hot-standby-routing-protocol
https://networklessons.com/cisco/ccie-routing-switching/introduction-gateway-redundancy