Section 9.2: Hot Standby Router Protocol (HSRP)

Hot Standby Router Protocol addresses the problem caused by first-hop failures generally having static default gateway addresses on hosts. Previously, a failure at the default gateway address would leave the host unable to communicate outside of its own subnet. With HSRP, a set of routers work together to represent a single virtual standby router. A failure of the active router would result in a switch to the standby router, and packets would continue to be forwarded. Cisco routers use HSRP, which enables end stations to continue communicating throughout the network even when the default gateway becomes unavailable. The standby router group functions as a single router configured with a virtual IP and MAC address, distinct from the physical routers in the network. Because the routers in the standby group route packets sent to a virtual address, packets are still routed through the network even when the router originally forwarding the packets fails.

If the primary or lead router of a group of HSRP routers fails, a standby router in the same group begins to forward traffic for the HSRP group. The routers decide within the group which router forwards traffic for the virtual address. At regular intervals, the routers exchange information to determine which routers are still present and able to forward traffic. When routers are configured to be part of an HSRP group, the routers recognize their own native MAC address, as well as the HSRP group MAC address. Routers whose Ethernet controllers only recognize a single MAC address will use the HSRP MAC address when performing as the active router and the burn-in address (BIA) when in standby mode or not speaking.

9.2.1: HSRP Group Members

The HSRP group consists of an active router, a standby router, a virtual router, and other routers. To facilitate load sharing, a single router may be a member of multiple HSRP standby groups on a single subnet. Each standby group emulates a single virtual router. However, there is a limit of 255 standby groups on any given LAN.

Note: Some platforms do not support multiple HSRPs because of the single MAC address per interface restriction. You can lift this restriction by using the standby use-bia command.

9.2.2: Addressing HSRP Groups Across ISL Links

HSRP routers can provide for redundancy and load sharing across the same subnet. As of Cisco IOS 11.3, HSRP routers can also provide for redundancy and load sharing across different subnets. For each standby group, an IP address and a single well-known MAC address with a unique group identifier is allocated to the group. The IP address of a group is in the range of addresses belonging to the subnet in use on the LAN. However, the IP address of the group must differ from the addresses allocated as interface addresses on all routers and hosts on the LAN, including virtual IP addresses assigned to other HSRP groups.

Running HSRP over ISL allows users to configure redundancy between multiple routers that are configured as front ends for VLAN IP subnets. By configuring HSRP over ISLs, users can eliminate situations in which a single point of failure causes traffic interruptions. This provides improvement in overall networking resilience by providing load balancing and redundancy capabilities between subnets and VLANs. To configure HSRP over an ISL link between VLANs, you must define the encapsulation type; configure the IP address; and enable HSRP.