HSRP allows multiple routers to share a virtual IP and MAC address so that hosts on a network don’t detect or realize a failure if one occurs on the primary gateway router. Here are some features of HSRP:
- The Virtual IP and Virtual MAC are active on the HSRP Active router
- Standby routers listen for Hellos from the Active router, this is defaulted to 3 second hello interval with a 10 sec dead timeout
- Higher priority configured on an HSRP router determines the Active router, preemption is disabled by default, the priority range is between 1-255, the default priority value is 100
- HSRP supports tracking, and the tracked objects can be tied to a priority where if an interface that is being tracked fails it will decrease the priority on that router.
- There can be multiple HSRP groups configured on an interface up to 255
- The Virtual MAC address is always 0000.0c07.acXX, where XX is the hex conversion of the HSRP group
- The Virtual IP configured must be in the same subnet as the router interface it’s applied to on the LAN
- The Virtual IP must be unique from any other device in the subnet or interfaces configured on the two redundant routers
- HSRP supports clear text and MD5 authentication
HSRP has one Active router at a time, non-Active routers are considered to be in an idle state. To provide load balancing between the two redundant routers, Multiple HSRP or MHSRP can be configured with multiple HSRP groups for each subnet, and assigning priority for the various groups to prefer one router or the other to be Active and allows for this load balancing. To further support this method of load balancing the DHCP configuration would define one router as the default gateway and the other half would reference the other router as the default gateway, and they both would failover to each other.
HSRP is Cisco proprietary.