Diagnosing IP Networks › How to Trace a TCP/IP Route

How to Trace a TCP/IP Route

It is possible to obtain a hop-by-hop record of the route taken by a packet through a network, starting from the source host and finishing at the destination host.

You can perform a traceroute as follows:

• Directly from the TCP/IP : Network Diagnosis Functions menu
• Use the traceroute action against a host on a list or display; for example on a connection list or the IP Node Monitor
• Issue the TRACEROUTE command from the command line

Perform a Traceroute From the Network Diagnosis Functions Menu

A traceroute attempts to trace the route from the issuing point through all hosts and routers to the destination host; otherwise it traces the route to the point at which a break in communication has occurred.

To trace a route to a remote host

1. Enter /IPDIAG at the prompt.

   The TCP/IP : Network Diagnosis Functions menu appears.

2. Select TR - Trace Route and complete the following field:
   • Host Name/Addr

     Specifies the name or IP address of the remote host you want to access.

     If you want to use a specific address of the local stack interface as the issuing point, select it in the Source Address field.

     Press Enter.

     The TCP/IP : Trace Route Result List panel appears.

Use Traceroute Action

The Trace Route Result List panel displays the results of the traceroute action. This display is progressively updated as information arrives.

The most likely reason for performing a traceroute action is that a ping is unable to reach a remote host or it is indicating slow response times in the TCP/IP network.

If the ping reported that the host is unreachable, you can do one of the following:

• Perform a traceroute to find out where the failure is occurring
• Check the Routing Table

If the host is unreachable, the traceroute might return a result such as Hop Count Exceeded, Network Unreachable, or Host Unreachable.

If the hop count is exceeded and the IP address column on the last entry or entries is an *, the indication is that a device is not responding (possibly the destination host). If this is the case, or if the result indicated that the network or host was unreachable, you may want to examine the Routing Table or Interfaces for the last listed hop. To do this, apply I (Interfaces) or R (Routing Table) against the host or router recorded as the last hop on the list.

When using the Routing Table action, your TCP/IP management region uses masking to ensure that the presented Routing Table contains only those addresses that are relevant to the path you are tracing. This is a network address mask based on the class of address being traced. For example, if you are tracing a route to the foreign host 192.168.2.66, your region inserts the mask 192.168.2.*.

If the hop count is exceeded and the same number of hops is shown as was set in the Hop Limit field, change the hop limit to a larger number and reissue the traceroute by pressing F6 (Action).

If the ping indicates a slow response, you can check for the following in your traceroute results:

• Check the Notes column to see if there are any outstanding alerts for the node. If the IP Node Monitor is monitoring the node, you can see if any alerts are outstanding.
• Check the trip times—these should increase at a steady rate the further along the route the packets are sent. If there is a sudden and marked increase in the trip time for a particular hop along the route, it might indicate that this is where the problem is occurring. To further investigate:
  • Apply the I (Interfaces) action to the host or router at the hop where the problem is occurring and check the Interfaces for clues to the performance degradation
  • Apply the TN (Telnet) action to start a Telnet connection to the host or router where the problem is occurring and investigate the configuration or other possible causes
• Check the hop list—if there is no evidence of slow trip times, look at the hop list to check whether an unexpected route is being used.

Note: To analyze response time problems, use the Transaction Path Analyzer facility.

If the traceroute does not return any abnormal results, use the reporting facility to check for large file transfers.