First-hop redundancy protocol

First hop redundancy protocols (FHRP) are a category of networking protocols designed to protect the default gateway used on a subnetwork by allowing two or more routers to provide backup for that address.^[1][2][3] In the event of failure of an active router, the backup router will take over the address, usually within a few seconds. In practice, such protocols can also be used to protect other services operating on a single IP address, not just routers.

Examples of such protocols include (in approximate order of creation):

• Hot Standby Router Protocol (HSRP) - Cisco's initial, proprietary standard developed in 1998^[4]
• Virtual Router Redundancy Protocol (VRRP) - an open (albeit patent-encumbered) standard protocol based largely on Cisco's HSRP
• Common Address Redundancy Protocol (CARP) - patent-free, unencumbered alternative to Cisco's HSRP and the IETF's VRRP, developed in October 2003
• Extreme Standby Router Protocol (ESRP) - Extreme Networks' proprietary standard with fast failover and layer 2 protection
• Gateway Load Balancing Protocol (GLBP) - a more recent proprietary standard from Cisco that permits load balancing as well as redundancy
• Routed Split multi-link trunking (R-SMLT) - an Avaya redundancy protocol
• NetScreen Redundancy Protocol (NSRP) - a Juniper Networks proprietary router redundancy protocol providing load balancing
• Chassis Cluster Redundant Ethernet - a Juniper Networks proprietary Ethernet redundancy protocol, used on its SRX platform^[5]
• Multi-active Gateway Protocol (MAGP) - a Mellanox proprietary protocol based on VRRP that allows active-active operation^[6]