Recognize High availability (FHRP)

Exam: Cisco 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)

In this chapter we will discuss how to recognise the high availability (FHRP). This is an important topic under the topic of IP services. The FHRP is the full form of first hop redundancy protocol. It is nothing but a group of protocols that are designed in such a way that they help a router to take over another one in case of failures. It basically protects the default gateway that is used in a sub network. In a simple language if the main router fails the backup router will start to function in a matter of few seconds. There are many types of FHRP but we will be discussing mainly three types of FHRPin this chapter. A shared network is bound to have a lot of problems. In the past many solutions have been suggested but nothing has worked better than the FHRP. This is exactly why most of the networks use this today.


VRRP stands for Virtual router redundancy protocol. It was clearly defined by Cisco in RFC 2338. The VRRP is supported today by many vendors across the world. The VRRP has the unique ability to use multicast IP addresses to effectively communicate with others. It has been assigned the IP protocol number 112. The VRRP can support up to 255 groups. The VRRP has not been accepted officially yet by the IETF standard. It is also reffered to as an open standard protocol. With the help of VRRP the router can get automatic assignment of IP routers. This can increase the numbers of the routing paths. This protocol creates virtual routers that help it to fulfil the purpose. This allows the master and the backup routers to act as a group. The default gateway is automatically is assigned to the virtual router and not the physical router. Only when the physical router fails then automatically another physical router is selected. The physical router is also called the master router in this case. The interesting point to be kept in mind is that the VRRP gives information on the state of the router and not on the route that is exchanged by the router. The VRRP does have its limitations. Its scope can extend only to a single subnet. The VRRP cannot affect the routing table in any way and it does not promote the IP routes either, Cisco has been trying to patent this particular protocol for some time now.


The initial protocol that was developed by Cisco was the (HSRP) also called the Hot Standby Router protocol. It is an effective fault tolerant default gateway that was successfully designed by Cisco. It was described initially in the RFC 2281. This protocol can successfully set a framework that can activate the default gateway in a situation where the primary gateway fails. The HSRP is not a typical routing protocol. It does not affect the routing table in anyway. The HSRP can also work when more than one interface fails in a router. It is indeed a Cisco proprietary standard. This provides a method to replace the default gateway that is absolutely reliable and cannot be disturbed by any events. This also does not need the devices to run any particular software. The HSRP can send a hello message to a multicast address. It does so using a UDP 1985 port. The primary router in this case will act as the virtual router and will have a pre defined gateway. If the primary router fails then the next router with the highest priority will take over and it goes on. This will ensure that there is connectivity from the head to the end.


It is called the gateway load balancing protocol. This is a new concept that was introduced by Cisco. This allows load balancing along with redundancy. This was basically an attempt by Cisco to address the problems faced in the other routers. This adds the basic load balancing function to the Cisco proprietary protocol. The GLBP allows setting priorities on different gateways. It also allows weighing the parameters before they are set. The load balancing is not based on the traffic load alone. It depends on the number of hosts that are present in the default gateway router. The GLBP will balance the load in a round robin manner. Round robin is a term that is used to refer a pattern where the items are processed in a sequence. The pattern is circular and begins again from the start. The GLBP functions in a very simple manner it will select one AVG (active virtual gateway) for each group. The others in the group will act as a backup in case the AVG fails. If in a situation two members fail then the second best AVG is brought into action. The others are placed in the listening status. This whole process is controlled using the hold time and hello timers. These timers are set at three and ten seconds each for a better result. The AVG that is selected will automatically allot MAC address to all the members of the GLBP group. There can be up to four AVFs that are used at the same time. The GLBP is designed in such a way that by default they use the local multicast addresses. They use UDP 3222 port to function. The GLBP has the ability to weight each GLBP router. The GLBP will place weight on each device to calculate the load that is being shared. Each GLBP router will clearly advertise the weight and the assignment. The GLBP is used only when a large circuit is used in a primary router. The higher weight enables it to take twice more traffic compared to the primary server.

We hope that this chapter on recognize high availability (FHRP) will help you to understand the topic better and do well in your exam. It is critical from the point of view of the exam and do prepare the topic well.

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