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Passing Cisco's CCNA and CCNP Exams: Traceroute |
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In preparation for your CCNA and CCNP exam success, you've
got to learn to troubleshoot Cisco routers. And while ping
is a great basic IP connectivity tool, it doesn't give you
all the information you need to diagnose network connectivity
issues.
Let's say you have six routers between CityA and CityB. You
send a ping from A to B, and get this return:
R1#ping 172.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.1.1.1, timeout is 2
seconds:
.....
Success rate is 0 percent (0/5)
The five periods indicate that there is no IP connectivity
to CityB. Problem is, that's about all ping tells you. You
can have 5 or 50 routers between the two points, so how can
you tell which downstream router has the problem?
That's where traceroute comes in. Traceroute sends three
datagrams with a Time To Live (TTL) of 1. Those datagrams
will timeout once they hit the first router in the path, and
that router will respond with an ICMP Time Exceeded message.
In response, the sending router sends three more datagrams,
but these have a TTL of 2. This means that the next router
in line will send back ICMP Time Exceeded messages. This process
continues until the final destination (CItyB) is reached the
output of the command shows us the path the data took:
Router1#traceroute 271.1.1.1
Type escape sequence to abort.
Tracing the route to 271.1.1.1
1 20.1.1.1 4 msec 4 msec 4 msec
2 30.1.1.1 20 msec 16 msec 16 msec
3 271.1.1.1 16 msec * 16 msec
How does this help troubleshoot a problem? Let's say that
the second router in this path, 30.1.1.1, doesn't know how
to get to 271.1.1.1. The output would look like this:
Router1#traceroute 271.1.1.1
Type escape sequence to abort.
Tracing the route to 271.1.1.1
1 20.1.1.1 4 msec 4 msec 4 msec
2 30.1.1.1 20 msec 16 msec 16 msec
3 * * *
This indicates that the router at 30.1.1.1 doesn't know how
to get to the final destination. Now you have a better idea
of which router has an issue!
Now here's the bad part: you're going to get 30 lines of
three asterisks, and until you abort this traceroute, you're
going to just watch those asterisks go across the screen.
There's an abort sequence that the router mentions in the
first line of the console output, but the router doesn't tell
you what it is! So I will - this top-secret sequence is TWICE,
one right after the other.
That keystroke takes a little getting used to, but a CCNA
or CCNP can do it! Add this command to your Cisco skill set,
and it will serve you well both on the CCNA and CCNP exams
and your real-world networks. And you'll impress your friends
by knowing how to stop a traceroute!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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