How to resolve EIGRP problems

Certification: Cisco CCIE Routing and Switching - Cisco Certified Internetwork Expert Routing and Switching

EIGRP is referred to asEnhanced Interior Gateway Routing Protocol. EIGRP is a Cisco propriety protocol which is designed to operate on Cisco switches and routers. It can work on small, medium and large level networks as well with efficiency. In its metric calculation, EIGRP utilizes bandwidth and delay. However, it can also make use load, reliability and maximum transmission unit (MTU) for this purpose. The technicality of EIGRP allows it to be termed as an advanced distance-vector routing protocol as well as a hybrid routing protocol. EIGRP ensures the smooth flow of the network by gathering information from its surroundings and providing the perfect route for transmission. Moreover, EIGRP also carries out the task of storing all the information regarding the topologies of all the networks. Furthermore, EIGRP only works with routers and switches which have been made by Cisco.

However, there are some issues which hinder the performance of EIGRP. These issues lead to unusual routing of data packets and might also prevent data from reaching its destination. It is important to identify these issues and troubleshoot them in an efficient manner. Following are some of the main problems associated with EIGRP and ways in which they can be resolved.

Connection is not being established with neighboring router

This is one of the most common problems in which an EIGRP router is not able to establish a connection with its neighboring router. This problem in communication prevents the router to devise an appropriate path for the data packets to be transferred over the network, leading to delay in transfer of data. In order to solve this issue, you need to carry out some checks and then resolve the issue accordingly.

Firstly, you need to check if all of the routers are connected together on the same network. It is important that all of the routers are on the same networks as EIGRP will not allow routers to communicate and neighbors to be formed if they are on different networks. If two routers are not on the same network, you must bring them on the same one. Once done, you then need to check if both of the routers have been configured with the same MTU (Maximum Transmission Unit), which determines the largest size of data packet which could be transferred. If they are not, then you need to adjust the settings to configure them on the same MTU. In this step, if the problem is not resolved, you might also need to check the physical layer 2 connections to see if they are fine. All of these problems are essential to be resolved because neighboring routers will not be able to communicate if the following issues persist.

EIGRP Routing Table issues

EIGRP routing table consist of all the information regarding the position of routers and the topology of the network. This enables the EIGRP to identify the correct router on which the data has to be transmitted. Any issues with the EIGRP routing tables could lead to the data being sent over wrong routes and not reaching its destination.

If all of the routes are missing from the routing table, the problem could lead to the data packets being lost over the network as their route could not be determined. In this case, you should check whether the deleted routes lie in the topology table as well or not. If yes, then you need to check the physical cabling to resolve this issue. The next step which you need to do is to make sure that all the routers are assigned different IDs on the routing table. If more than one router has been assigned the same routing ID, it will not be identified uniquely on the routing table and routing decisions will not be made appropriately. The “show ip EIGRP topology” command will let you access the routing IDs of all the routers on the network.You should modify the list immediately to resolve this issue.

You should also keep in mind that discontinuous networks do not work if auto-summarization is enabled. Hence, if the network is discontinuous, you should disable auto-summarization so that the network can establish connectivity. Furthermore, you should also make sure that the MTU assigned on each interface is the same so that no data packets are lost in the process.

Redistribution issues

In the process of redistribution, the router starts redistributing a protocol other than EIGRP. This is not really a problem but it becomes one when the correct way for redistribution is not followed. Hence, if this problem arises, there is one thing which you are immediately supposed to do. You need to make sure that a default metric has been specified for all the redistributed routes. If a default metric for redistributed routes has not been configured, you will face a lot of problems. You could make use of the “show ip EIGRP topology” command to examine into this issue and then resolve it.

Issue of Access Control Lists

The Access control lists are used to make sure that the data packets are filtered when they pass through a router. However, these lists interfere with the process of EIGRP. In this regard, you should access the ACLs and immediately and disable them to check if the problem is eliminated. If the problem is finished then you know that ACLs are interfering with EIGRP’s process. In this regard you will have to reconfigure the ACLs. Please note that permanently disabling the access control lists is not a solution as this could lead to further problems. We simply disabled it to confirm if it was causing the issue. Once you have checked, you must go through the process of ACL configuration to ensure it does not interfere with EIGRP or cause any further issues.

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