In computer networking, the Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. Version 1 of the protocol was described in in 1998. Version 2 of the protocol includes improvements and supports IPv6 but there is no corresponding RFC published for this version.
The protocol establishes an association between gateways in order to achieve default gateway failover if the primary gateway becomes inaccessible. HSRP gateways send multicast hello messages to other gateways to notify them of their priorities (which gateway is preferred) and current status (active or standby).
Operation
The primary router with the highest configured priority will act as a virtual router with a pre-defined gateway IP address and will respond to the ARP or ND request from machines connected to the LAN with a virtual MAC address. If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same MAC address, thus achieving transparent default gateway failover.
{| class="wikitable"
! HSRP version || IP protocol !! Group address !! UDP port !! Virtual MAC address range
|-
| 1 || IPv4 || 224.0.0.2 (all routers) || 1985 || 00:00:0c:07:ac:XX
|-
| rowspan=2|2 || IPv4 || 224.0.0.102 (HSRP)