Content

| 7.1 |  | Troubleshooting the Application Layer |
| 7.1.3 |  | Eliminating Layer 4 |

Layer 4 is the home of UDP and TCP protocols and is not as easy to eliminate.

For example, assume there are FTP connection problems. To troubleshoot Layer 4, use the following steps:

1. Use the show access-list command. Are there any access-lists that could be stopping traffic? Notice which access lists have matches.
2. Clear the access-list counters with the clear access-list counters command and try to establish an FTP connection again.
3. Verify the access-list counters. Have any increased? Should they increase?

Improperly configured access lists are common problem areas. Be sure the implications of each access list statement are understood. This may sound strange but it sometimes helps to think like the packet.

However, if the access lists are functioning as expected, then the problem must lie in a higher layer.