Information Technology Reference
In-Depth Information
counters were reset, reported at the top of show interface . Generally, the counters can be looked at as
an accurate percentage of packets received or sent. If the counters have not been reset in a long time, or
if a problem is suspected, the counters should be reset using clear counters command, and a new reading
should be taken after a reasonable period of time has elapsed. If a problem is detected on a WAN or
dialup link, refer to Part IV. If a problem is detected on a LAN connection, see the section
“Troubleshooting Physical Connectivity Problems,” earlier in this chapter.
Check for ACLs
Check this router for any access lists applied to an interface using ip access-group , or any other firewall
or packet filters configured. Does the packet filtering permit the desired source/destination to
communicate using the requested protocol? If you are unsure, see the section “Troubleshooting
Upper-Layer Problems.”
Check for Network Address Translation
Check to see if this router is configured for network address translation. If it is, is it supposed to translate
packets between the source and destination? Has it been configured correctly?
At this point, you will want to move on to one of the next-hop routers. Record routers that you have
already visited on a piece of paper. Also record any problems or questions that arose at the router. This
record will help you detect routing loops and will provide useful information if you find it necessary to
call for support.
Troubleshooting Upper-Layer Problems
Even though there may be IP connectivity between a source and a destination, problems may still exist
for a specific upper-layer protocols such as FTP, HTTP, or Telnet. These protocols ride on top of the basic
IP transport but are subject to protocol-specific problems relating to packet filters and firewalls. It is
possible that everything except mail will work between a given source and destination. Before
troubleshooting at this level, it is important to first establish whether IP connectivity exists between the
source and the destination. If IP connectivity exists, then the issue must be at the application layer.
The following could go wrong:
A packet filter/firewall issue might have arisen for the specific protocol, data connection, or return
traffic.
The specific service could be down on the server.
An authentication problem might have occurred on the server for the source or source network.
There could be a version mismatch or incompatibility with the client and server software.
Generic
To troubleshoot an upper-layer protocol connectivity problem, you must understand how it works. You
can generally find this information in the latest RFC for the protocol or on the developer's web page.
Questions that you should answer to make certain that you understand the protocol include these:
What IP protocols does the protocol use (TCP, UDP, ICMP, IGMP, or other)?
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