HSRP

HSRP Overview

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

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.

HSRP Preemption

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.

R1 has been configured with the HSRP priority of 150 while R2 has the default HSRP priority of 100. Preemption has been enabled on R1. With a higher priority, R1 is the active router and R2 is the standby router. Due to a power failure affecting only R1, the active router is no longer available and the standby router, R2, assumes the role of the active router. After power is restored, R1 comes back online. Because R1 has a higher priority and preemption is enabled, it will force a new election process. R1 will re-assume the role of the active router and R2 will fall back to the role of the standby router.

HSRP States and Timers

A router can either be the active HSRP router responsible for forwarding traffic for the segment, or it can be a passive HSRP router on standby, ready to assume the active role if the active router fails. When an interface is configured with HSRP or is first activated with an existing HSRP configuration, the router sends and receives HSRP hello packets to begin the process of determining which state it will assume in the HSRP group.

The table summarizes the HSRP states.

HSRP State	Description
Initial	This state is entered through a configuration change or when an interface first becomes available.
Learn	The router has not determined the virtual IP address and has not yet seen a hello message from the active router. In this state, the router waits to hear from the active router.
Listen	The router knows the virtual IP address, but the router is neither the active router nor the standby router. It listens for hello messages from those routers.
Speak	The router sends periodic hello messages and actively participates in the election of the active and/or standby router.
Standby	The router is a candidate to become the next active router and sends periodic hello messages.

The active and standby HSRP routers send hello packets to the HSRP group multicast address every 3 seconds by default. The standby router will become active if it does not receive a hello message from the active router after 10 seconds. You can lower these timer settings to speed up the failover or preemption. However, to avoid increased CPU usage and unnecessary standby state changes, do not set the hello timer below 1 second or the hold timer below 4 seconds.

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