Learning by Doing: CISCO Certified Network ... - SCN Research

One last layer 3 tool…you can also turn your router into a mini-layer 3 protocol
inspector with “debug” commands. Be careful when using these because they
save all their information in RAM/DRAM. Too much information can *choke*
out the performance of your router so only use debug commands sparingly. To
view ping packets (aka ICMP packets) use the debug ip icmp command. You
should see something like this.
Randy#debug ip icmp
ICMP packet debugging is on
Randy#
01:02:29: ICMP: time exceeded (time to live) sent to 192.168.3.2 (dest
was 192.168.4.2)
01:02:29: ICMP: time exceeded (time to live) sent to 192.168.3.2 (dest
was 192.168.4.2)
01:02:29: ICMP: time exceeded (time to live) sent to 192.168.3.2 (dest
was 192.168.4.2)
01:02:29: ICMP: dst (192.168.3.1) port unreachable sent to 192.168.3.2
01:02:31: ICMP: dst (192.168.3.1) port unreachable sent to 192.168.3.2
01:02:32: ICMP: dst (192.168.3.1) port unreachable sent to 192.168.3.2
Randy#
**Don’t forget to use undebug all or undebug ip icmp when you are finished.**
4. Finally Telnet (terminal emulation), an application layer program, tests the
functionality of all 7 layers. If you can telnet from one router to another, then
everything should be working fine and you won’t need anything from this lab.
Here is an example of using telnet from Randy to Ward. You should see:
Randy#telnet 192.168.30.2
Trying 192.168.30.2 ... Open
User Access Verification
Password:
Ward>
One problem with telnet: if a vty password is not “set” on the other router you
will not be able to access the router, even though everything is working fine.
Let’s look at what you will see if you do not have a vty password set:
Randy#telnet 192.168.30.2
Trying 192.168.30.2 ... Open
Password required, but none set
[Connection to 192.168.30.2 closed by foreign host]
Randy#
194