Compare HSRP, VRRP, and GLBP as first hop redundancy protocols. What are the key differences, and when would you choose each?

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First Hop Redundancy Protocols provide gateway redundancy so that if the router serving as the default gateway for a subnet fails, another router seamlessly takes over. Without FHRP, a router failure leaves all hosts on the subnet without a path to other networks until the default gateway is manually changed or the failed router recovers.

HSRP, the Hot Standby Router Protocol, is a Cisco proprietary protocol that is the most widely deployed FHRP in Cisco environments. Two or more routers form an HSRP group sharing a virtual IP address that hosts use as their default gateway. One router is elected as the active router, handling all traffic for the virtual IP, while the other serves as the standby router, ready to take over if the active router fails. The active router is selected based on priority, with the highest priority winning. Preemption can be enabled so that a higher-priority router reclaims the active role after recovering from a failure. HSRP operates in active-standby mode, meaning only the active router forwards traffic and the standby router's bandwidth is wasted during normal operation. HSRP version 2 supports IPv6, increases the number of groups to 4096, and uses a different multicast address than version 1.

VRRP, the Virtual Router Redundancy Protocol, is the IETF standard equivalent of HSRP defined in RFC 5798. It operates similarly with a master router and backup routers sharing a virtual IP address. The key difference is that VRRP is vendor-neutral and supported by virtually all enterprise router and switch platforms. VRRP allows the virtual IP address to be the same as a physical interface address on the master router, which saves an IP address. VRRP uses preemption by default, unlike HSRP where preemption must be explicitly enabled. Like HSRP, VRRP is active-standby, meaning only the master forwards traffic for the virtual IP.

GLBP, the Gateway Load Balancing Protocol, is another Cisco proprietary protocol that adds load balancing to gateway redundancy. Unlike HSRP and VRRP where only one router is active, GLBP allows multiple routers to actively forward traffic for the same virtual IP address. GLBP uses a single virtual IP but assigns different virtual MAC addresses to each participating router. When a host sends an ARP request for the virtual IP, the Active Virtual Gateway responds with the MAC address of one of the Active Virtual Forwarders, distributing hosts across the available routers. This means all routers handle traffic during normal operation, utilizing the bandwidth of every gateway router rather than wasting the standby capacity.

Choosing between these protocols depends on your environment and requirements. In a pure Cisco environment, HSRP is the default choice for its simplicity and deep integration with Cisco features like interface tracking and Enhanced Object Tracking. In a multi-vendor environment, VRRP is the only practical choice because it is the standard supported by all vendors. When you need to utilize the bandwidth of multiple gateway routers rather than leaving standby capacity idle, GLBP is the appropriate choice, though it is limited to Cisco platforms. For most modern enterprise designs, VRRP provides the best balance of standardization and functionality, with the vendor-neutral advantage becoming increasingly important as organizations adopt multi-vendor strategies and network automation tools that benefit from protocol standardization.

All three protocols support interface tracking, where the FHRP priority is reduced if an upstream interface fails, triggering a failover even when the gateway router itself is healthy. This is essential for detecting failures in the network path beyond the gateway router.