First Hop Redundancy Protocol Explained

If two devices are connected via a single link, they have a single point of failure. If the link fails, the connected devices fail to connect. To deal with this type of situation, additional links are used. An additional link serves as a backup. It only works when the main link fails. Additional links are known as redundant links. In other words, a redundant link is a backup link that is used when the main link fails. Similarly, a redundant device is a backup device that is used when the main device fails. The process of using backup devices and links is known as redundancy.

Since redundancy requires additional equipment, it should be used when the cost of the additional equipment is less than the cost of damage from network failure. Based on the requirement, a network can add redundancy to all components, some components, or a few critical components. If a network adds redundancy, it needs additional protocols to manage redundant devices and links. FHRP is a set of protocols used to manage redundant gateway routers.

Basic concepts of FHRP

A gateway router connects the local network to remote networks. If a network uses multiple gateway routers, it must configure a redundancy protocol. A redundancy protocol manages multiple gateway routers. It automatically replaces a failed gateway router with a functional backup gateway router. For example, you add an additional gateway router to a network. The following image shows the network before and after adding the backup router.

Without FHRP, you have the following options.

• Reconfigure all hosts to use the backup router if the main gateway router fails.
• Configure some hosts to use the new gateway router. If the current gateway router fails, configure the hosts that use it to use the new gateway router.
• If the new gateway router fails, configure the hosts configured to use it to use the current gateway router.

All of the above options require manual action if an outage occurs. Outages are unpredictable. To detect an outage, you must monitor the network around the clock. When an outage occurs, you must configure all hosts to use the backup default gateway. You also need to reconfigure all hosts to use the original setting after the outage.

In a small network, you can use these options with a few extra efforts. However, using these in a large network is a nightmare. For example, a medium-sized network has 1000 hosts on a local subnet. You must configure and reconfigure all these hosts every time an outage occurs. Nobody wants to handle this complicated and tedious task. Additionally, since the change is made on the host system, the host system also remains unavailable to the user while you make the change.

An FHRP protocol makes this task easy. To configure an FHRP protocol, you have to make the following changes.

• Create a virtual IP address on the main gateway router
• Create the same virtual IP address on the backup gateway router
• Configure an FHRP protocol on both routers to respond to the virtual IP address
• Configure all hosts to use the virtual IP address as the gateway router IP address

Once an FHRP protocol is configured, you do not need to continuously monitor the gateway router. The configured FHRP protocol will automatically replace the down gateway router with the available gateway router. For example, if the main gateway router is down, traffic will be diverted to the backup router. If the backup router is down or the main router is up again, it will automatically reroute traffic back to the main router.

After implementing FHRP, you do not need to make any changes to the host systems. It means there will be no downtime on host systems due to the gateway router configuration.

FHRP protocols

FHRP is not a single protocol. It is a class of protocols that provides similar functions. For Cisco certification programs, three FHRP protocols are relevant. These protocols are HSRP (Hot Standby Router Protocol), VRRP (Virtual Router Redundancy Protocol), and GLBP (Gateway Load Balancing Protocol). HSRP is the first FHRP protocol developed by Cisco. Cisco developed this protocol as a proprietary protocol. When this protocol became popular, the IETF developed an open-source version of its. It was the VRRP protocol. In other words, the VRRP protocol is the open-source copy of the HSRP proprietary protocol. Later, Cisco developed a more robust protocol. It was GLBP. GLBP is the enhanced, updated version of HSRP.

Conclusion

First Hop Redundancy Protocols (FHRP) provide critical network reliability by ensuring continuous gateway availability without manual intervention. By implementing protocols such as HSRP, VRRP, or GLBP, you can minimize downtime, reduce manual reconfiguration, and enhance network resilience. Understanding and applying FHRPs is essential for designing robust, highly available network infrastructure.