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Sep 22

Basic EIGRP Troubleshooting Steps

EIGRP Troubleshooting

Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) are routing protocols that are very different and have their unique advantages and disadvantages. Enhanced Interior Gateway Routing Protocol (EIGRP) is a hybrid protocol of both OSPF and RIP that takes the best parts of both protocols and creates a protocol that inherits advantages from both. EIGRP is an in-between of the simple RIP and the more complex OSPF and as such, some common issues might cause EIGRP to not work properly. There are some troubleshooting steps a network technician can take to diagnose and repair these problems. An important concern in EIGRP is neighbour flapping, or the failure of a neighbour relationship, which can result in higher bandwidth use and increased CPU utilisation.

One of the most common issues when using EIGRP is when neighbour relationships are not properly formed. A neighbour check can be done, with the first step being to ensure that the neighbouring routers are attached to the same primary network as EIGRP will not form networks with secondary networks. Next, pinging between neighbours with packet sizes up to the interface MTU size can be done to check if there are cabling problems, interface problems or Layer 2 problems. The MTU setting for both interfaces should be the same.

An EIGRP redistribution check can also be done to ensure that the redistribution router is not distributing routing protocols other than IGRP or EIGRP into EIGRP that are not expected in the EIGRP topology table. The command show ip eigrp topology net mask can help to verify the expected networks in the EIGRP topology table. If there are other routing protocols that are not EIGRP or IGRP into EIGRP, a default metric must also be specified.

An EIGRP route check can also be done to verify that the routing table is populated with EIGRP routes and is not missing external routes. This can be done with the show ip route eigrp and show ip route eigrp commands.

Similar to OSPF troubleshooting, the technician will also have to ensure that there are no ACLs interfering with router communications and should disable any ACLs that could cause problems. Misconfigured hello and hold intervals can also cause neighbours to flap. The commands ip hello-interval eigrp and ip hold-time eigrp  are used to set the hello and hold intervals respectively on both neighbouring routers. The hold interval should be at least 3 times the hello timer to reduce the risk of neighbour flapping. Even if the intervals are correctly configured, an overly congested link can also cause hello packets to be lost.

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