This tutorial explains basic concepts and options of the First Hop Redundancy Protocol. Learn what the Layer 3 redundancy is and how FHRP provides redundancy.
What is redundancy?
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 deal with this type of situation, additional links are used. An additional link acts as a backup link. 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 additional equipment is less than the cost of damage caused by 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 group of protocols that are used to manage redundant gateway routers.
Basic concepts of FHRP
A gateway router connects the local network to remote networks. If a network wants to use multiple gateway routers in such a way that the hosts automatically connect to the available gateway router, it has to configure a redundancy protocol. A redundancy protocol manages multiple gateway routers. It automatically replaces a failed gateway router with a functional backup gateway router.
Let us take an example. For backup purposes, a network administrator adds an additional gateway router to the network. The following image shows the network before and after adding the backup router.
Now suppose, the administrator does not know anything about FHRP. Without FHRP, the administrator may use the following options.
- Make no change in the current host configuration and 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 are configured to use the current gateway router to use the new gateway router.
- Similarly, if the new gateway router fails, configure the hosts that are configured to use the new gateway router to use the current gateway router.
All of the above options require manual action if an outage occurs. The outages are unpredictable. To detect an outage, the administrator needs to monitor the network round the clock. When an outage occurs the administrator needs to configure all hosts to use the backup default gateway router. The administrator also needs to reconfigure all hosts to use the original setting after the outage.
In a small network, these options may work. But in a large network, these options will not work. For example, assume that a medium-sized network has 1000 hosts in a local subnet. The administrator must configure and reconfigure all these hosts every time an outage occurs. No administrator would like 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 the administrator makes the change.
An FHRP protocol makes this task very easy and simple. To configure an FHRP protocol, the administrator needs 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, the administrator does not need to monitor the gateway router at all times. 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, it will divert the traffic to the backup router. If the backup router is down or the main router is up again, it will automatically divert the traffic back to the main router.
The following image shows how an FHRP protocol works.
After the FHRP implementation, the administrator also does not need to make any changes on the host systems. It also means there will be no downtime on host systems due to the gateway router configuration.
The three FHRP options
As mentioned earlier, FHRP is not a single protocol. It is a class of protocol that provides similar functions. For Cisco certification programs, you should study three FHRP protocols. 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, IETF developed an open-source version of this protocol. 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 and updated version of the HSRP.
That's all for this tutorial. In the next part of this article, we will discuss the HSRP protocol in detail. If you like this tutorial, please don’t forget to share it with friends through your favorite social network.
