Hot Standby Router Protocol

HSRP or Hot Standby Router Protocol is a protocol defined by Cisco and now described in rfc2281.

Purpose of HSRP

The purpose of HSRP is to ensure network connectivity in case of Router or access circuit failure, by having one or more standby Router(s) waiting to take over from the failing active Router.

Other high availability Router protocols

• GLBP Gateway Load Balancing Protocol
• VRRP Virtual Router Redundancy Protocol

How does HSRP work HSRP works by two or more Routers agreeing upon which Router serves the virtual Router. The Virtual Router The Virtual Router is a MAC-address and a IP Address the active Router serves beside its configured IP address. If the active Router fails one of the standby Routers becomes the Virtual Router by serving the virtual MAC-address and IP Address. HSRP example In picture 1 three Routers R1,R2 and R3 are setup in a HSRP Group. The Virtual IP address 10.0.0.1 and virtual MAC-Address 00-10-0C-07-AC-0A are served by the active Router R1. R1 continuesly transmits hello packets to the standby Routers. If the Active Router fails one of the standby Routers will become the active Router and start serving the Virtual IP address 10.0.0.1 and the virtual MAC-address 00-10-0C-07-AC-0A. There are no load balancing between the Routers.

Picture 1: HSRP group with Router R1 active and Router R2 and R3 as standby Routers

Configuration of R1, R2 and R3

hostname R1 ! interface FastEthernet1/0 ip address 10.0.0.11 255.0.0.0 standby priority 130 standby preempt standby ip 10.0.0.1

hostname R2 ! interface FastEthernet1/0 ip address 10.0.0.12 255.0.0.0 standby priority 120 standby preempt standby ip 10.0.0.1

hostname R3 ! interface FastEthernet1/0 ip address 10.0.0.13 255.0.0.0 standby priority 110 standby preempt standby ip 10.0.0.1