FHRP, short for First Hop Redundancy Protocol, is used at the customer premises for route redundancy. Usually, you have a default gateway defined to your ISP (in case of connecting to the public Internet) or to your server (in the event you have an internal network). If the default gateway router fails, you will not be able to connect to the external resources unless a replacement is found and necessary configuration changes are made. First Hop Redundancy protocols will allow default gateway redundancy. With two or more default gateways defined using FHRP, in the event of a router failure there’s a backup router that will take place of the failed gateway, transparent to the user. The user’s host computer is configured with a virtual IP address and the switch between the redundant routers takes place without any user intervention. A typical configuration is shown in the figure below:
There are 3 types of FHRP protocols that are widely used:
Of the above three types of FHRPs, HSRP is Cisco proprietary protocol, where as VRRP is a standards based protocols. The two protocols HSRP (short for Hot Standby Router Protocol) is very similar to VRRP (Virtual Router Redundancy Protocol), except that VRRP is based on Open standards. When using HSRP, one router assumes the function of “Active” router and the other is known as “Standby” router. The routers communicate within themselves and when the active router fails, the standby router will kick-in. There can be more than two routers in a HSRP group.
GLBP, short for Gateway Load Balancing Protocol, is a bit different from HSRP and VRRP, in the sense, GLBP offers load balancing among the redundant gateways. However, note that HSRP and VRRP too offer load balancing using distinct “groups”, but load balancing is not native to HSRP and VRRP, where as GLBP, as the name implies, offers load balancing inherently.
FHRP is not to be confused with defining multiple gateway routers with a host’s network configuration file. A host computer may be defined with one or more default gateways, so that if one fails the other default gateway takes its position. In this case, the default gateways have different IP addresses, whereas in FHRP, the default gateway will have only one virtual IP address that is to be configured at the workstation.
Check out the router network simulator for practicing HSRP and VRRP labs.
Figure courtesy: https://www.engineerkhan.com/switch/hsrp-concept-notes/
HSRP – No longer for the weak of heart