In this series of troubleshooting tickets, we will find and fix problems with the OSPF adjacency between routers.

At the end of the lesson, you can download the EVE-NG file and try to troubleshoot the ticket yourself. Then, you can review the lesson and see if you used the same approach.

Ticket Topology

For this ticket, we are going to use the following initial topology. 

All devices run OSPF in Area 0 and must establish adjacency with their neighboring routers.

Ticket Issues

Suppose you are a network engineer supporting a big enterprise company. You have received a ticket with the following two reported issues:

• Issue #1 - R3 cannot form OSPF adjacency with R5.
• Issue #2 - DSW1 cannot form OSPF adjacency with DSW2.

You are given the "cisco/cisco" credentials. However, they do not have the highest privilege level, so you cannot use the show run command. Therefore, you must resolve the issues without looking at the routers' configurations.

Troubleshooting OSPF Adjacency Issues

Okay, let's start with the troubleshooting process. What is the workflow for troubleshooting an OSPF adjacency problem?

Some educators unnecessarily overcomplicate the troubleshooting process, and students have the impression that it is something complex that you need to have "talent" to do. In reality, it is pretty simple and straghtforward. You just execute a few show commands on both sides and compare them, as shown in the diagram below. That's it. 

The process is simply to compare the physical and OSPF settings of two connected interfaces. You execute show ip ospf interface on both sides, compare the output, and find what is mismatched. 99% of the time, that's what troubleshooting OSPF adjacency problems look like. You just need to know what to look for.

I always break down the troubleshooting workflow into two main steps.

• First, I ensure that general interface settings like port status, ACL, or MTU are working correctly.
• Then, I verify the OSPF-specific settings of the interfaces on both sides.

Step 1 - Interface Settings

Remember that the OSPF neighborship works per interface. So, I always start by verifying the general IP settings of the interface connecting to the neighbor we troubleshoot. The first things that I always check are:

The easiest way to check all these facts in exam environments is to use the show ip interface command. It gives a long output containing all the information we need. However, in the real world, engineers typically use a combination of show run and show ip interface brief to crosscheck the same information.

In the end, I always throw some pings between the routers that must become neighbors. This ensures that there is IP connectivity, and the problem is most likely in the OSPF configuration rather than in the network segment between the devices.

Step 2  -  OSPF Settings

The OSPF settings that prevent two routers from becoming neighbors are not that many. The following table summarizes the requirements that must be met.

It is also a good practice to check the device's syslog. Routers generate syslog messages to report the most common OSPF misconfiguration. In exam environments, the logging may be intentionally disabled to make troubleshooting more difficult. However, in real-world networks, it is always good practice to check the syslog.

Issue #1

With his troubleshooting workflow in hand, let's go ahead and start with our first issue.