Information Technology Reference
In-Depth Information
General IP Troubleshooting Suggestions
This chapter approaches the process of troubleshooting TCP/IP connectivity issues with the assumption
that you will have access to the client (or source) and may not have access to the server (or destination).
If the problem is determined to be a server issue, you contact the server administrator. If you are the
server administrator, you can apply the troubleshooting process in reverse (server to client) to further
troubleshoot connectivity issues. This chapter will not address the specifics of troubleshooting
server-side IP services; for this, consult the manual or web page for the software or service running on
the server.
Because TCP/IP does not store path information in its packets, it is possible for a packet to have a
working path from the source to the destination (or vice versa), but not to have a working path in the
opposite direction. For this reason, it may be necessary to perform all troubleshooting steps in both
directions along an IP path to determine the cause of a connectivity problem.
Narrowing Down the Problem Domain
To efficiently troubleshoot a TCP/IP connectivity problem, it is necessary to identify a single pair of
source and destination devices that are exhibiting the connectivity problem. When you've selected the
two devices, test to make sure that the problem is actually occurring between these two devices.
Possible problems include these:
•
Physical layer issue somewhere along the path
•
First-hop Layer 3 connectivity issue, local LAN segment
•
Layer 3 IP connectivity issue somewhere along the packet's path
•
Name resolution issue
Where to start:
1.
Try to ping from the source to destination device by IP address. If the ping fails, verify that you are
using the correct address, and try the ping again. If the ping still fails, go to the next section,
“Troubleshooting Local Connectivity Problems.” Otherwise, proceed to Step 2.
2.
Try to ping from the source to the destination device by name. If the ping fails, verify that the name
is correctly spelled and that it refers to the destination device, and then try the ping again. If the ping
still fails, go to the section “Troubleshooting Domain Name Server Problems,” later in this chapter.
Otherwise, proceed to Step 3.
3.
If you can ping the destination by both name and address, it appears that the problem is an
upper-layer problem. Go to the section “Troubleshooting Upper Layer Problems,” later in this
chapter.
Troubleshooting Local Connectivity Problems
This section describes how to troubleshoot local connectivity problems on LAN segments such as
Ethernet or Token Ring. Going through the methodology in this chapter with help determine and resolve
problems moving packets on the local LAN segment or to the next-hop router. If the problem is
determined to be past the local LAN segment, then you will be referred to the section “Troubleshooting
IP Connectivity and Routing Problems,” later in this chapter. If the source device is connected via a
modem, then you should consult Chapter 16, “Troubleshooting Dialup Connections.”
Possible problems include these:
