The CCNA Basic: Traceroute explained

CCNA 200-301

CCNP Enterprise

CCNP Security

CCIE Enterprise Lab

CCIE Security Lab

CCNP Service Provider

CCNP Data Center

CCNP Collaboration

CCIE DC Lab

2019-12-25

Traceroute is a command-line interface-based tool that identifies the path used by packets to reach their destination. The tool also uses ICMP messages, but unlike ping, it identifies each router in the path used by the packet. Traceroute is useful when troubleshooting network problems because it helps you determine what the problem is. You can determine which router in the path to the unreachable target should be examined more carefully, which may be the cause of network failure.

Traceroute sends a series of ICMP echo request packets to the destination. The lifetime (TTL) parameter for the first series of messages is set to 1, which means that the first router in the path will discard the packet and send the ICMP timeout message. Then increase the TTL by 1 until you reach the target host and receive the ICMP echo reply message. The original host can then use the ICMP message received to identify all routers in the path.

NOTE

The traceroute command on Windows is named tracert. On Unix and Cisco IOS traceroute, use the traceroute command to call it.

Here is an example of using the tracer command in Windows:

In the above output, you can see that the traceroute command lists the IP addresses of all routers in the path.

Traceroute on Unix-like operating systems

The Traceroute command on Unix is slightly different from the Windows version. It uses UDP packets with large target port numbers (33434 to 33534), which are unlikely to be used by any application on the target host. Similar to the Windows version of the command, traceroute on Unix uses TTL to get the IP address of the intermediate router. When it reaches the target host, it will reply to the ICMP port unreachable message.

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