Cisco provides HSRP and HSRP for IPv6 as a way to avoid losing outside network access if your default router fails.
HSRP is a Cisco-proprietary FHRP that is designed to allow for transparent failover of a first-hop IP device.
HSRP ensures high network availability by providing first-hop routing redundancy for IP hosts on networks configured with an IP default gateway address. HSRP is used in a group of routers for selecting an active device and a standby device. In a group of device interfaces, the active device is the device that is used for routing packets; the standby device is the device that takes over when the active device fails, or when pre-set conditions are met. The function of the HSRP standby router is to monitor the operational status of the HSRP group and to quickly assume packet-forwarding responsibility if the active router fails.
HSRP Priority and Preemption
The role of the active and standby routers is determined during the HSRP election process. By default, the router with the numerically highest IPv4 address is elected as the active router. However, it is always better to control how your network will operate under normal conditions rather than leaving it to chance.
HSRP priority can be used to determine the active router. The router with the highest HSRP priority will become the active router. By default, the HSRP priority is 100. If the priorities are equal, the router with the numerically highest IPv4 address is elected as the active router.
To configure a router to be the active router, use the standby priority interface command. The range of the HSRP priority is 0 to 255.
By default, after a router becomes the active router, it will remain the active router even if another router comes online with a higher HSRP priority.
To force a new HSRP election process to take place when a higher priority router comes online, preemption must be enabled using the standby preempt interface command. Preemption is the ability of an HSRP router to trigger the re-election process. With preemption enabled, a router that comes online with a higher HSRP priority will assume the role of the active router.
Preemption only allows a router to become the active router if it has a higher priority. A router enabled for preemption, with equal priority but a higher IPv4 address will not preempt an active router. Refer to the topology in the figure.
The physcial network topology shows three PCs connected to a switch. The switch in turn is connected to two routers, router R1 with IP address 172.16.10.2/24 which is the currect active router with a priority of 150, and router R2 with IP address 172.16.10.3/24 which is the current standby router with a priority of 100. R1 and R2 both connect to a backbone cloud. There is a virtual router with a virtual IP address of 172.16.10.1/24 and a virtual MAC address of 0000.0C07.AC01.