Hot Standby Router Protocol

The Hot Standby Router Protocol (HSRP) provides high network availability by routing IP traffic from hosts without relying on the availability of any single router. HSRP is used in a group of routers to select an active router and a standby router. The 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 other preset conditions are met.

HSRP is useful for hosts that do not support a router discovery protocol (such as Internet Control Message Protocol [ICMP] Router Discovery Protocol [IRDP]) and that cannot switch to a new router when their selected router reloads or loses power.
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.

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 User Datagram Protocol (UDP)-based hello packets to detect router failure and to designate active and standby routers. Below is an example of an HSRP topology.

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

To enable the HSRP on an interface, we can use the following command:
Router(config-if)# standby [group-number] ip [ip-address
[secondary]]

To configure the time between hello packets and the hold time before other routers declare the active router to be down, use the following command:

Router(config-if)# standby [group-number] timers [msec]
hellotime [msec] holdtime

To set the Hot Standby priority used in choosing the active router. The priority value range is from 1 to 255, where 1 denotes the lowest priority and 255 denotes the highest priority:

Router(config-if)# standby [group-number] priority priority