Cisco Certified Expert

Hot Standby Routing Protocol HSRP

HSRP is not a TCP/IP standard defined by an RFC; instead, it is a Cisco protocol that provides automatic router backup. HSRP allows one router to assume the function of a second router if the second router fails. HSRP is useful for critical networks that need a failover router for network reachability. HSRP uses a priority scheme to determine the default active router. The default priority is 100. If you configure a router with a higher priority, it becomes the active router if both routers are configured at the same time. If a second (or third) router is configured after an HSRP router is already active, and that new router is configured with a higher priority, it does not take over the active role unless it's configured with the preempt command.

A standards-based protocol similar to HSRP called Virtual Router Redundancy Protocol (VRRP) is defined in the proposed RFC 2338. VRRP is not an exam blueprint and is not covered in this book.

HSRP is supported over Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) interfaces. Although each router has its own physical MAC address and logical IP address, with HSRP they share a virtual MAC address and IP address. The active router assumes the virtual IP (VIP). The default gateway of hosts is configured with the VIP address. After a failure, the standby router takes over the active role. HSRP can also be configured to track an outgoing interface (usually a serial interface); if the interface fails, the active router resigns, and the standby router becomes the active router.

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