8-6. Router Redundancy with HSRP

· Route processors in the same or another chassis can share redundant gateway addresses on a VLAN by using the Hot Standby Router Protocol (HSRP).
· Route processors sharing a common HSRP IP address must belong to the same HSRP group number.
· The HSRP address appears on the network with a special virtual MAC address00-00-0C-07-AC-XX, where XX is the HSRP group number (0 to 255). The hosts on the HSRP VLAN use this MAC address as the default gateway.
· Although HSRP is enabled on an interface, each route processor still maintains its own unique IP and MAC addresses on the VLAN interface. These addresses are used by other routers for routing protocol traffic.
· When an HSRP group is enabled, the highest-priority HSRP device at that time becomes the active router, whereas the second-highest-priority stays in the standby state. All other HSRP devices in the group maintain a "listening" state, waiting for the active device to fail. A new active router election occurs only when the active device fails. The previous active router (having the highest priority) may reclaim its active role by preempting the other HSRP routers in the group.
· HSRP devices communicate by sending a hello message over UDP at multicast address 224.0.0.2. These messages are sent every 3 seconds by default.
· Devices on a VLAN use the HSRP address as their default gateway. If one of the HSRP devices fails, there will always be another one to take its place as the default gateway address.