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| 5.1 |  | Troubleshooting Network Layer Problems |
| 5.1.2 |  | Isolating the problem methodology |

Although this module will describe many common problems and solutions for troubleshooting various routing protocols, it is important to note that there is no single template for solving these Layer 3 problems. Routing problems are solved with a good methodical process, using a series of commands to isolate and diagnose the problem.

Many of the examples in this module are just that, examples. It is important to be able to apply these examples and concepts to other routing protocols and other scenarios.

The first step is to isolate the problem. This can be done by identifying and documenting the symptoms.

Here are some areas to explore when diagnosing a possible problem involving routing protocols:

General Network Issues:
Many times a change in the topology, such as a down link, may have other affects on other areas of the network which might not be obvious at the time. This may include the installation of new routes, static or dynamic, removal of other routes, and so on.

Some of the things to consider include:

- Has anything in the network changed recently?
- Is there anyone currently working on the network infrastructure?

Connectivity Issues:
Check for any equipment and connectivity problems including:

- Power problems: outages, intermittent problems, environmental problems such as overheating, and so on.
- Layer 1 problems: cabling problems, bad ports, service provider problems, and so on.

Some commands, which can be used to check these and other issues, include:

- ping
- traceroute
- show interface, show ip interface

Neighbor issues – If the routing protocol establishes an adjacency with a neighbor, check to see if there are any problems with the routers forming neighbor relationships.

Topology database – If the routing protocol uses a topology table or database, check the table for anything unexpected, such as missing entries or unexpected entries.

Routing table – Check the routing table for anything unexpected, such as, missing routes or unexpected routes. Use debug commands to view routing updates and routing table maintenance.

Many troubleshooting issues and solutions are common amongst different routing protocols. At times, this could be due to whether the routing protocol is a distance-vector or link state routing protocol, if the routing protocol uses AS or process-id numbers, whether or not the routing protocol forms neighbor relationships, is the routing protocol classful or classless, and whether or not the routing protocol has a topology database. For example, split horizon issues will be common with all IGP distance-vector routing protocols, RIP, IGRP, and EIGRP. With IGRP and EIGRP, their AS numbers must be the same for routers to exchange updates, whereas with OSPF the process-id number is local only to that router. In this module, some issues described on one routing protocol may also apply to others.

| Lab Activity Lab Exercise: Correcting Problems at the Network Layer In this lab, the student will use various Cisco commands to correct network problems.