Cisco Systems 78-11741-02 manual Configuring the Hot Standby Router Protocol, IPC-100

Configuring IP Services

Configuring the Hot Standby Router Protocol

Configuring the Hot Standby Router Protocol

The Hot Standby Router Protocol (HSRP) provides high network availability because it routes IP traffic from hosts on Ethernet, FDDI, or Token Ring networks without relying on the availability of any single router. HSRP is used in a group of routers for selecting an active router and a standby router. (An active router is the router of choice for routing packets; a standby router is a router that takes over the routing duties when an active router fails, or when preset conditions are met.)

HSRP is useful for hosts that do not support a router discovery protocol (such as ICMP Router Discovery Protocol [IRDP]) and cannot switch to a new router when their selected router reloads or loses power. Because existing TCP sessions can survive the failover, this protocol also provides a more transparent recovery for hosts that dynamically choose a next hop for routing IP traffic.

When the HSRP is configured on a network segment, it provides a virtual MAC address and an IP address that is shared among a group of routers running HSRP. The address of this HSRP group is referred to as the virtual IP address. One of these devices is selected by the protocol to be the active router. The active router receives and routes packets destined for the MAC address of the group. For n routers running HSRP, n + 1 IP and MAC addresses are assigned.

HSRP detects when the designated active router fails, at which point a selected standby router assumes control of the MAC and IP addresses of the Hot Standby group. A new standby router is also selected at that time.

Devices that are running HSRP send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers.

Previously, when HSRP was configured on an interface, ICMP redirect messages were disabled by default. With Cisco IOS Release 12.1(3)T, ICMP redirection on interfaces configured with HSRP are enabled by default. See the “Enabling HSRP Support for ICMP Redirect Messages”section later in this document for more information.

You can configure multiple Hot Standby groups on an interface, thereby making fuller use of redundant routers and load sharing. To do so, specify a group number for each Hot Standby command you configure for the interface.

Note Token Ring interfaces allow up to three Hot Standby groups each, the group numbers being 0, 1, and 2.

Cisco IOS IP Configuration Guide

IPC-100