Information Technology Reference
In-Depth Information
Check the Configuration
Verify that the interface on the device is configured properly and is not shut down. If the device is
connected to a hub or switch, verify that the port on the hub or switch is configured properly and is not
shut down. Check both speed and duplex.
Check the Network Interface
Most interfaces or NICs will have indicator lights that show whether there is a valid connection; often
this light is called the link light. The interface may also have lights to indicate whether traffic is being
sent (TX) or received (RX). If the interface has indicator lights that do not show a valid connection,
power off the device and reseat the interface card.
Troubleshooting IP Connectivity and Routing Problems
When troubleshooting IP connectivity problems across large networks, it always helps to have a network
diagram handy so that you can understand the path that the traffic should take and compare it to the path
that it is actually taking.
When IP packets are routed across a network, there is the potential for problems at every hop between
the source and the destination, so test connectivity at each hop to determine where it is broken is the
logical troubleshooting methodology.
The following could be wrong:
•
A router may not have a route to the source or destination.
•
The network might have a routing loop or other routing protocol-related problem.
•
A physical connectivity problem might have occurred.
•
A resource problem on one router might be prohibiting proper router operation. This could possibly
be caused by lack of memory, lack of buffers, or lack of CPU.
•
A configuration problem might have occurred on a router.
•
A software problem might have occurred on a router.
•
A packet filter or firewall might be preventing traffic from passing for an IP address or protocol.
•
An MTU mismatch problem might have occurred.
Determining Where to Start
The most detailed method to find a problem would obviously be to start at the next hop away from the
source and work your way one hop at a time toward the destination, exploring all possible paths along
the way. You would then test basic IP connectivity and possibly protocol connectivity from each router
forward. Although in some cases this method is the only one available, the process can generally be
shortened by first performing a traceroute from the source to the destination to determine the first
problematic hop. If the traceroute method does not provide an answer, you will have to fall back to the
longer method.
