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Overview: Traceroute basics

Goal: Visualize how to use traceroute to troubleshoot a  broken path when ping fails.
Topology: Host H1 and server S1 (google.com) are connected by 2 routers R1, R2. R2's F0/1 is down.
Stepss: 1) H1 ping google.com fails. Run traceroute to troubleshoot the problem,
             2) At H1, traceroute sends the first Probe. R1 responds. It's the first hop.
             3) Send the second Probe. R2 responds. It's the second hop.
             4) Send the third Porbe. No response. The ping problem is at the thrid hop.

Ping and traceroute

We use ping to check a remote site. However, if ping fails, how to locate the trouble spot?
Traceroute is a tool to troubleshoot broken path problems
This animation shows how to use ping and traceroute to find and isolate path problems.

H1 ping goolge.com fails

H1 ping google.com. It prints the following messsage in H1's command window:
     H1#ping www.googlee.com
     Sending 2 ICMP ping fails. When ping timeout, H1 prints a "." at the command line.
Ping is dropped by R2. When Ping timeout, H1 prints a "."
     H1#ping www.googlee.com
     Sending 2 ICMP ping fails. When ping timeout, H1 prints a "." at the command line.
    .

Traceroute: First hop reachable (R1)

- At H1, run "Trceroute google.com"  It prints a message and sends a Probe to chekc the first hop.
     H1# traceroute S1
     traceroute to S1 (3.3.3.2), 30 hops max, 40 byte packets
- R1 receives Probe and replies a message. When H1 receives this response, it prints 
     1  IP.R1.1 (1.1.1.254)  0.553 ms          It means that R1 is reachable and the RTT is 0.553 milisecond.
Note：In Windows, traceroute is called tracert.

Second hop reachable (R2)

H1 sends the second Probe to check the second hop. R2 replies a message. 
When H1 receives this reply, it prints 
       2  IP.R2.1 (2.2.2.254)  1.363 ms
Now H1 has learned that the first two hops are recheable and they are R1 and R2.

Third hop unreachable

H1 sends anther Probe to check the third hop.
When R2 receives Probe, it is unable to froward it to google.com since F0/1 is down. R2 drops Probe.
As a result, H1 won't receive reply from the thrid Probe.

Ping failure is isolated

When traceroute timeout, it knows that the path to google.com is broken at the third hop.
He can pass this lead to network admin to fix it.
Note: To learn more of traceroute mechanism, see "7. Traceroute details". It shows how to Probe works progressively to test one next hop a time. It also descibes the structure of Probe (UDP) and Response (ICMP).