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Using traceroute for Troubleshooting and Verification

by Jeremy L. Gaddis on September 12, 2011 · 0 comments

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Previously, we discussed using the ping command. Another useful utility in our toolbox is traceroute.

The traceroute command is another diagnostic tool, used for displaying the path that IP packets take through a network to the destination.

Like ping, the traceroute utility is built into Microsoft Windows (called “tracert”), Apple OS X, and Linux. It’s also available on Cisco routers and switches.

Assume that our network topology looked like this:

Network Diagram

What should be obvious is that there is only one path available for packets to get from R1 to R4. If, on R1, we ran a traceroute to R4, the output would look similar to this:

R1# traceroute 172.16.34.4

Type escape sequence to abort.
Tracing the route to 172.16.34.4

  1 172.16.12.2 9 msec 9 msec 8 msec
  2 172.16.23.3 16 msec 17 msec 17 msec
  3 172.16.34.4 25 msec 25 msec *

If you match up the IP addresses shown in the output from traceroute to our topology diagram, you can see that the packets take the (only) path from R1 to R2 to R3 to R4.

Often, though, our networks aren’t this simple. Consider the network topology shown in the picture at the top of this article.

How might IP traffic get routed from Seattle to Tampa? Just from looking at the diagram, it’s impossible to know because, as you can see, there are several different paths that it could take. The traceroute command lets us know the exact path that the traffic is taking through the network.

NOTE: In the real world, security people like to block traffic that allows traceroute to function properly. In these cases, the line of output will simply read “* * *”.

Lab Exercise

  • How Do We Get There From Here? (Coming Soon)

Summary

Although the traceroute command has several options that we can use, we’ve stuck to the basics for now. You’ll use this command extensively once you get into routing and then you’ll see just how useful it can be.

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