Information Technology Reference
In-Depth Information
Ruling Out Duplicate IP Addresses
To rule out a duplicate IP address, you can disconnect the suspect device from the LAN or shut down
the suspect interface and then try pinging the device from another device on that same LAN segment. If
the ping is successful, then there is another device on that LAN segment using the IP address. You will
be able to determine the MAC address of the conflicting device by looking at the ARP table on the device
that issued the ping.
If at this point you still do not have local connectivity for either the source or the next-hop router,
proceed to the next section.
Troubleshooting Physical Connectivity Problems
This section describes how to troubleshoot Layer 1 and 2 physical connectivity issues on LANs such as
Ethernet or Token Ring. For troubleshooting information on dialup links or WAN connections, consult
the chapters in Part IV, “Troubleshooting Serial Lines and WAN Connections.”
Even though it may seem logical to first troubleshoot at the physical layer, problems can generally be
found more quickly by first troubleshooting at Layer 3 and then working backward when a physical
problem is found or suspected.
Possible problems include these:
•
Configuration is incorrect.
•
Cable is faulty or improperly connected.
•
Wiring closet cross-connect is faulty or improperly connected.
•
Hardware (interface or port) is faulty.
•
Interface has too much traffic.
Rule Out a Configuration Problem
Check to make sure that all cables are connected to the appropriate ports. Make sure that all
cross-connects are properly patched to the correct location using the appropriate cable and method.
Verify that all switch or hub ports are set in the correct VLAN or collision domain and have appropriate
options set for spanning tree and other considerations.
Check Cable Connections
Verify that the proper cable is being used. If this is a direct connection between two end systems (for
example, a PC and a router) or between two switches, a special crossover cable may be required. Verify
that the cable from the source interface is properly connected and is in good condition. If you doubt that
the connection is good, reseat the cable and ensure that the connection is secure. Try replacing the cable
with a known working cable. If this cable connects to a wall jack, use a cable tester to ensure that the
jack is properly wired. Also check any transceiver in use to ensure that it is the correct type, is properly
connected, and is properly configured. If replacing the cable does not resolve the problem, try replacing
the transceiver if one is being used.
