|  | 4.4 | Basic Networking Testing
| 4.4.4 |
Testing the network layer with the trace command

The trace command is the ideal tool for finding where data is being sent in your network. The trace command is similar to the ping command, except that instead of testing end-to-end connectivity, trace tests each step along the way. This operation can be performed at either the user or privileged EXEC levels. The trace command takes advantage of the error messages generated by routers when a packet exceeds its Time To Live (TTL) value. The trace command sends several packets and displays the round-trip time for each. The benefit of the trace command is that it tells which router in the path was the last one to be reached. This is called fault isolation. In this example, we are tracing the path from York to Rome. Along the way the path must go through London and Paris. If one of these routers had been unreachable, you would have seen three asterisks (*) instead of the name of the router. The trace command would continue attempting to reach the next step until you escaped using the Ctrl-Shift-6 escape sequence.
| Lab Activity
| In this lab you will use the IOS traceroute command. The traceroute command uses ICMP packets and the error message generated by routers when the packet exceeds its Time To Live (TTL).