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| 6.2 |  | Implementing Router Redundancy in a Switched Network |
| 6.2.3 |  | Hot Standby Router Protocol (HSRP) |

One way to achieve near 100-percent network uptime is to use HSRP (RFC 2281). HSRP provides network redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first-hop failures in network edge devices or access circuits.

By sharing an IP address and a MAC address, a set of two or more routers can operate as a single router called a virtual router. This set is known as an HSRP group or a standby group. A single router elected from the group is responsible for forwarding the packets that hosts send to the virtual router. To minimize network traffic, only the Active and Standby routers send periodic HSRP messages once the protocol has completed the election process. If the Active router fails, the Standby router takes over as the Active router. If the Standby router fails or becomes the Active router, then another router is elected as the Standby router. Hosts continue to forward IP packets to a consistent IP and virtual MAC address and the changeover between routes is transparent to the end workstation.

On a particular LAN, multiple hot standby groups may coexist and overlap. Each standby group emulates a single virtual router. The individual routers may participate in multiple groups. In this case, the router maintains separate states and timers for each group. Each standby group has a single virtual MAC and IP address.