When the active router going fails for any reason, the First Hop Redundancy Protocol (FHRP) changes the standby router as an active router. When the active router is going fails for any reason and the standby router stops seeing Hello messages from the forwarding router.
Then the standby router assumes the role of the forwarding router because it assumes both the IP and MAC addresses of the virtual router and, the host devices see no disturbance in service. This process is possible due to the First Hop Redundancy Protocols (FHRPs). The difference FHRPs are the following:-
Host Standby Router Protocol (HSRP)
It is a Cisco-proprietary First Hop Redundancy Protocol (FHRP) planned to allow for transparent failover of a first-hop IPv4 device. It allows configuring two or more routers as standby routers and only a single router as an active router at a time which ensure high network availability by providing first-hop routing redundancy for IPv4 hosts on networks configured with an IPv4 default gateway address.
All the routers in a single HSRP group share a single MAC address and IP address, which acts a default gateway to the local network. It selects an active router as well as a standby router in the group.
The active roster is responsible for forwarding traffic to the network and sends the hello packet to the standby router. If it fails and the Standby router doesn’t receive a hello packet for three specific times, it takes up all the responsibilities of the active router and forwards the traffic.
HSRP for IPv6
This is also Cisco-proprietary FHRP same as HSRP. The difference is only that HSRP is working in an IPv4 environment and HSRP for IPv6 is working in IPv6 environments. An HSRP IPv6 group uses a virtual MAC address resulting from the HSRP group number and a virtual IPv6 link-local address.
Periodic router advertisements (RA) message would be sent for the HSRP virtual IPv6 link-local address when the HSRP group is active. When the group becomes inactive for any reason the Router Advertisement (RAs) messages stop after a final RA is sent when the group would be leaving the active state.
Virtual Router Redundancy Protocol Version 2 (VRRPv2)
Virtual Router Redundancy Protocol (VRRP) is an election protocol that provides redundancy to routers within a Local Area Network.
It is a non-proprietary protocol that allows several routers on a multi-access link to use the same virtual IPv4 address. VRRP was designed to remove a single point of failure in a static default-route environment by dynamically assigning virtual IP routers to participate hosts.
In a VRRPv2 configuration, one router is elected as the virtual router master, and other routers acting as backups, in case the virtual router master fails.
A virtual router is a collection of physical routers whose interfaces must belong to the same IP subnet. A virtual router ID is assigned to each virtual router, but there is no restriction against reusing a VRID with a different address mapping on different LANs.
VRRPv3 supports IPv6 addresses including supports IPv4 addresses for dual-stack networks configured with VRRP or VRRP-E. It is compliant with RFC 5798. It is a faster switchover to backup devices than can be achieved using standard IPv6 neighbor discovery mechanisms.
With VRRPv3, a backup router can become a master router only in some seconds with less overhead traffic and no relations with the hosts. It is work in multi-vendor environments and is more scalable than VRRPv2.
Gateway Load Balancing Protocol (GLBP)
GLBP is the Cisco-proprietary FHRP. It is the abbreviation of the Gateway Load Balancing Protocol that protects data traffic from a failed router or circuit, like HSRP and VRRP. It is also allowing load sharing between a group of redundant routers.
GBLP is specially designed to overcome the limitations of HSRP and VRRP. The additional feature to Gateway Load Balancing Protocol is it can load share between the gateways whereas, in HSRP and VRRP, the standby routers are configured for redundancy and act as standby only and become active only when the active router fails.
GLBP for IPv6
It is also a Cisco-proprietary FHRP giving the same function as GLBP provides but in an IPv6 environment. It provides automatic router backup for IPv6 hosts configured with a single default gateway on a LAN. Multiple first-hop routers on the local area network join to offer a single virtual first-hop IPv6 router while sharing the IPv6 packet forwarding load.
ICMP Router Discovery Protocol (IRDP)
It is specified in RFC 1256, is a legacy FHRP. ICMP Router Discovery Protocol (IRDP) allows IPv4 hosts to locate routers that provide IPv4 connectivity to nonlocal IP networks.
It is using Internet Control Message Protocol (ICMP) router advertisements and router solicitation messages to allow a host to discover the addresses of operational routers on the subnet.