Deciphering Ping Outcomes

It is not sufficient to know that you don’t have IP connectivity towards the remote device, you have got to know why. Test Internet Speed is a great first step to network troubleshooting, but the outcomes are quite limited. As a CCNA and CCNP, you’ve got to know the way to diagnose the problem and resolve it. Just looking at the routing table isn’t sufficient – a high-powered Cisco debug, debug IP packet, can often show you exactly where the problem is.

WARNING: debug IP packet ought to not be run on any production router with no understanding the impact of this command in your router. This command outcomes in a great deal of output and can in fact lock up a router.

In this case, we’ll run the command on a residence lab router that can not ping 22.2.2.two. The debug will probably be turned on and a different ping sent.

R1#debug IP packet
IP packet debugging is on
R1#ping 22.2.2.two
Kind escape sequence to abort.
Sending 5, 100-byte ICMP Echoes to 22.2.2.two, timeout is 2 seconds:
3d23h: IP: s=1.1.1.1 (nearby), d=22.two.2.2, len 100, unroutable.
R1#undebug all
All potential debugging has been turned off

I’ve edited this output for clarity; the critical word is “unroutable”. This indicates that the packet isn’t leaving the router because there is no match in the routing table for this destination. We’ll configure a static default route and send the ping once again.

R1#ping 22.two.2.2
Kind escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 22.two.2.two, timeout is two seconds:
U.U.U
Good results rate is 0 percent (0/5)

That output may surprise those of you who are used to receiving 5 of exactly the same symbol back whenever you send a Ping Tests. We got three “U”s back together with two periods. We’ll now run debug IP packet and send the ping once again.

R1#debug IP packet
IP packet debugging is on
R1#ping 22.2.two.two
Form escape sequence to abort.
Sending 5, 100-byte ICMP Echoes to 22.two.two.two, timeout is two seconds:
3d23h: IP: s=172.12.123.1 (local), d=22.two.two.2 (Serial0), len 100, sending
R1#traceroute 22.2.2.two
Kind escape sequence to abort.
Tracing the route to 22.2.2.2
1 172.12.123.two 36 msec 36 msec 36 msec
two 172.12.123.two !H * !H
R1#undebug all
All probable debugging has been turned off

Once more, I’ve edited this output. The key word in this output is “sending”, which means that the packets are leaving the router. The ping return of “U.U.U” can be a general indication that the packets are indeed being transmitted, but that a downstream router is having a problem routing the packets. Operating traceroute reveals some a lot more exciting return characters! In this case, the downstream router did not have a match for the destination in its routing table.

It’s uncomplicated to concentrate on the nearby router when you’re not getting positive ping returns. When troubleshooting this type of problem, preserve in mind the issue might be on an intermediate router and not on the local router. Use debug IP packet to create confident the packets are leaving the local router, and traceroute to establish what downstream router could have the problem. And get utilized to the reality that pings and traceroutes can give you some unusual-looking returns!

Tags: nearby router, downstream router, Internet Protocol

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