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Cisco CCNA / CCNP Certification – Deciphering Ping Results

As you examine on your CCNA and CCNP exams, notably in case you’re getting hands-on apply in your house lab or rack rental service, you are going to be sending lots of pings. As a CCNA or CCNP candidate, that 5 exclamation factors (!!!!!) as a ping return signifies that you’ve IP connectivity to the distant vacation spot. 5 durations (…..) signifies that you just don’t have that connectivity.

It isn’t sufficient to know that you do not have IP connectivity to the distant machine, you have to know why. Ping is a good first step to community troubleshooting, however the outcomes are fairly restricted. As a CCNA and CCNP, you have to know diagnose the issue and resolve it. Simply wanting on the routing desk is just not sufficient – a high-powered Cisco debug, debug ip packet, can typically present you precisely the place the issue is.

WARNING: debug ip packet shouldn’t be run on any manufacturing router with out understanding the impact of this command in your router. This command leads to lots of output and might truly lock up a router.

On this case, we’ll run the command on a house lab router that can’t ping 22.2.2.2. The debug can be turned on and one other ping despatched.

R1#debug ip packet

IP packet debugging is on

R1#ping 22.2.2.2

Sort escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 22.2.2.2, timeout is 2 seconds:

3d23h: IP: s=1.1.1.1 (native), d=22.2.2.2, len 100, unroutable.

R1#undebug all

All potential debugging has been turned off

I’ve edited this output for readability; the essential phrase is “unroutable”. This means that the packet is just not leaving the router as a result of there isn’t any match within the routing desk for this vacation spot. We’ll configure a static default route and ship the ping once more.

R1#ping 22.2.2.2

Sort escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 22.2.2.2, timeout is 2 seconds:

U.U.U

Success charge is zero % (zero/5)

That output might shock these of you who’re used to getting 5 of the identical image again everytime you ship a ping. We acquired three “U”s again together with two durations. We’ll now run debug ip packet and ship the ping once more.

R1#debug ip packet

IP packet debugging is on

R1#ping 22.2.2.2

Sort escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 22.2.2.2, timeout is 2 seconds:

3d23h: IP: s=172.12.123.1 (native), d=22.2.2.2 (Serial0), len 100, sending

R1#traceroute 22.2.2.2

Sort escape sequence to abort.

Tracing the path to 22.2.2.2

1 172.12.123.2 36 msec 36 msec 36 msec

2 172.12.123.2 !H * !H

R1#undebug all

All potential debugging has been turned off

Once more, I’ve edited this output. The important thing phrase on this output is “sending”, which means that the packets are leaving the router. The ping return of “U.U.U” is a common indication that the packets are certainly being transmitted, however downstream router is having an issue routing the packets. Working traceroute reveals some extra fascinating return characters! On this case, the downstream router didn’t have a match for the vacation spot in its routing desk.

It is easy to focus on the native router while you’re not getting optimistic ping returns. When troubleshooting this sort of subject, remember the issue might be on an intermediate router and never on the native router. Use debug ip packet to verify the packets are leaving the native router, and traceroute to find out what downstream router might have the issue. And get used to the truth that pings and traceroutes may give you some unusual-looking returns!

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