Sunday, January 20, 2019

Traceroute command

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 the source host to the destination host and can be helpful in troubleshooting network problems. Use this command to find out which router in the path to an unreachable destination should be examined in more detail than the probable cause of the network outage.




In the picture above we can see that we have a network of four routers. The network is working properly. Consider what happens if we issue the traceroute command to 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.

Follow 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 routers in the path to the destination. Now suppose that the router R3 fails. Now think about the command set:
Rl # traceroute 172.16.0.2
Enter the escape sequence to cancel the operation.

Follow the route to 172.16.0.2
1 10.0.0.1 0 ms 0 ms 0 ms
2 10.0.0.1! H *! H
3 * *
Note that there is no response from R3 (192.168.5.2). With the help of this information we can assume that there is a problem with R3 and investigate the problem. H! The output indicates that the values ​​can not be accessed. Another character that can be displayed in the output command of traceroute is:
Number of milliseconds - round trip time in milliseconds.
* - The probe has taken out
A - Forbidden under administrative law (eg with an access list).
Q Source off (target is too busy).
The user has interrupted the test.
The U-port is not accessible.
The N network is unreachable.

The P-protocol is not reachable.
T -timeout.

 Unknown package type.

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