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:
- Use the show access-list command. Are there any
access-lists that could be stopping traffic? Notice which access lists have
matches.
- Clear the access-list counters with the clear access-list
counters command and try to establish an FTP connection again.
- Verify the access-list counters. Have any increased? Should they increase?
Improperly configured access lists are common problems. Be sure the
implications of each access list statement are understood.
However, if
the access lists are functioning as expected, then the problem must lie in a
higher layer.