VRRP / HSRP
acronymStands for: Virtual Router Redundancy Protocol (with Cisco's Hot Standby Router Protocol, HSRP)
VRRP and Cisco's HSRP: routers on a LAN share one virtual gateway address and MAC, so the default gateway outlives a router.
Hosts point at a virtual address held by whichever router is currently active; RFC 9568, the VRRP standard since April 2024, renamed the old Master the Active Router. Election runs on priority, the address owner at 255 always wins, and on failover the new Active Router announces the virtual MAC so switches and hosts follow it.
VRRP's virtual MAC is 00-00-5E-00-01-XX for IPv4 and 00-00-5E-00-02-XX for IPv6, XX being the VRID in hexadecimal, from IANA's OUI, with a VRID of 1 to 255. The Active Router answers ARP and Neighbor Solicitations with that address, never its own, and sources its advertisements from it so that bridges learn where it is. Advertisements go to 224.0.0.18 or ff02::12 as IP protocol 112 with TTL 255, every 100 centiseconds by default.
A Backup declares the Active Router down after three advertisement intervals plus a skew of (256 minus priority) times the interval over 256: about 3.6 seconds at the default priority of 100, and under 4 seconds at the default interval whatever the priority. Preemption is on by default. Authentication was removed because misconfigured secrets elected several Active Routers and no authentication stops a hostile node from acting as a second one; the TTL 255 check keeps attacks local.
HSRP is Cisco's protocol, published as RFC 2281 in March 1998 with Informational status, not as a standard. It uses UDP port 1985 to 224.0.0.2 with TTL 1, the virtual MAC 00-00-0C-07-AC-XX, XX being the group number, recommended Hello and hold times of 3 and 10 seconds, and preemption only when configured; Cisco's version 2 moves to 224.0.0.102 and groups 0 to 4095. Neither protocol watches the uplink, so vendors add tracking, which lowers a router's priority when an interface or route it depends on fails.
Also known as: vrrp, hsrp, fhrp, first hop redundancy, virtual gateway, virtual router
Sources
- RFC 9568, Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6, April 2024: sections 1, 1.1, 1.3, 1.7, 2.4, 5.1, 5.2, 6.1, 7.3, 8.1.2 and 9 (read 2026-10-19)
- RFC 2281, Cisco Hot Standby Router Protocol (HSRP), March 1998: sections 1, 5.1, 5.2 and 6.1 (read 2026-10-19)
- Cisco, First Hop Redundancy Protocols Configuration Guide, Configuring HSRP (Cisco IOS XE 16), updated June 3, 2026 (read 2026-10-19)