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| 7.1 |  | Troubleshooting the Application Layer |
| 7.1.2 |  | Eliminating Layers 1-3 |

When an application program cannot successfully connect to the destination host, establish at which layer the problem resides. Is it a lower layer problem or a higher layer problem?

For example, assume the problem is the inability to connect to a remote FTP server. To determine whether this is an application layer problem and not a lower layer problem, the first step is to verify Layer 3 connectivity. If successful, Layer 3 and lower can be eliminated as the source of the problem.

To troubleshoot use the following steps:

1. Ping the default gateway. If successful, Layer 1 and Layer 2 services are functioning properly.
2. Verify end-to-end (host-to-host) connectivity. Use an extended ping if attempting the ping from a Cisco router.

If these pings are successful, then Layer 1 through Layer 3 can be eliminated. Since they are functioning properly, the issue must exist at a higher layer.

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