The traceroute command on a Cisco device can be used to identify the path through which a packet reaches its destination. It identifies all routers in the path from source values to destination values and can be helpful in troubleshooting network problems. With this command, you can find out which router in the path to an inaccessible destination should be examined more closely as a possible cause of network failure.
Consider the following example networks:
Example of a Traceroute command network
In the picture above, we see that we have a network of four routers. The network works properly. Consider what happens when we issue the traceroute command on R1 to the IP address of the router's R4 Gi0 / 0 interface (172.16.0.2):
Rl # traceroute 172.16.0.2
Enter the escape sequence to cancel the operation.
Track the route to 172.16.0.2
1 10.0.0.1 0 ms 0 ms 0 ms
2 192.168.5.2 0 ms 0 ms 0 ms
3 172.16.0.2 0 ms 0 ms 0 ms
You can see that the traceroute command listed the IP addresses of all the routers in the path to the destination. Now assume that router R3 fails. Now check the command output:
Rl # traceroute 172.16.0.2
Enter the escape sequence to cancel the operation.
Track the route to 172.16.0.2
1 10.0.0.1 0 ms 0 ms 0 ms
2 10.0.0.1! HRS *! B
3 * *
Note that there is no response from R3 (192.168.5.2). Based on this information, we can assume that there is a problem with R3 and investigate the issue. The! H in the output indicates that the values are inaccessible. Other characters that can be displayed at the end of the traceroute command are:
Number of milliseconds - round trip time in milliseconds.
* - The probe has expired.
A - Administrative prohibited (eg with a call list).
Remove source source (target is busy).
Me - the user has canceled the test.
U-Port is not available.
The N network is inaccessible.
The P protocol cannot be reached.
T timeout.
? - Unknown package type.
Consider the following example networks:
In the picture above, we see that we have a network of four routers. The network works properly. Consider what happens when we issue the traceroute command on R1 to the IP address of the router's R4 Gi0 / 0 interface (172.16.0.2):
Rl # traceroute 172.16.0.2
Enter the escape sequence to cancel the operation.
Track the route to 172.16.0.2
1 10.0.0.1 0 ms 0 ms 0 ms
2 192.168.5.2 0 ms 0 ms 0 ms
3 172.16.0.2 0 ms 0 ms 0 ms
You can see that the traceroute command listed the IP addresses of all the routers in the path to the destination. Now assume that router R3 fails. Now check the command output:
Rl # traceroute 172.16.0.2
Enter the escape sequence to cancel the operation.
Track the route to 172.16.0.2
1 10.0.0.1 0 ms 0 ms 0 ms
2 10.0.0.1! HRS *! B
3 * *
Note that there is no response from R3 (192.168.5.2). Based on this information, we can assume that there is a problem with R3 and investigate the issue. The! H in the output indicates that the values are inaccessible. Other characters that can be displayed at the end of the traceroute command are:
* - The probe has expired.
A - Administrative prohibited (eg with a call list).
Remove source source (target is busy).
Me - the user has canceled the test.
U-Port is not available.
The N network is inaccessible.
The P protocol cannot be reached.
T timeout.
? - Unknown package type.