The trace user exec command discovers the routes that a router's packets follow when traveling to their destinations. The trace privileged exec command permits the supported IP header options to be specified, allowing the router to perform a more extensive range of test options.
The trace command works by using the error message generated by routers when a datagram exceeds its time-to-live (TTL) value. First, probe datagrams are sent with a TTL value of 1. This causes the first router to discard the probe datagrams and send back "time exceeded" error messages. The trace command then sends several probes and displays the round-trip time for each. After every third probe, the TTL is increased by one.
Each outgoing packet can result in one of two error messages. A "time exceeded" error message indicates that an intermediate router has seen and discarded the probe. A "port unreachable" error message indicates that the destination node has received the probe and discarded it because it could not deliver the packet to an application. If the timer goes off before a response comes in, trace prints an asterisk (*).
The trace command terminates when the destination responds, when the maximum TTL is exceeded, or when the user interrupts the trace with the escape sequence.
As with ping, it is a good idea to use the trace command when the network is functioning properly to see how the command works under normal conditions and so you have something to compare against when troubleshooting.
For detailed information on using the trace and extended trace commands, refer to the Cisco IOS Configuration Fundamentals Command Reference.
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