Breaking a traceroute

Unanswered Question
Sep 13th, 2007


I connect to router1 and then hop over to router2, I do a traceroute on router 2. However after hop 4 I get time outs and would like to kill the traceroute. The problem is when I hit ctrl_shift_6 and then x it puts me back on router 1. How can I kill the trace without ending back on router 1? As soon as I type in resume I am back on router 2 with the trace still going.



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robert.ward Thu, 09/13/2007 - 03:47

the normal keys ctrl-shift-6 drop you out.

if your putting x and hitting return it will then exit back to previous session or close your telnet session.

just do ctrl-shift-6 a few times if it doesnt drop you out promptly.

paul.matthews Thu, 09/13/2007 - 05:48

Put simply, you can't.

The escape sequence will always act on the first router yo are in - if you have multi hopped telnet to R1, then to R2, then to R3, then R4 you cannot selectively back out to R2 or R6 - the 6x sequence will put you back to R1.

If you want to be able to break into commands, you need to go "direct" to the router you want - it may be OK to telnet to a management system then hop from there, but hop via another router and you can only break into the first.


robert.ward Thu, 09/13/2007 - 05:55

hate to disagree but I ctrl break out of trace routes all the time and it doesnt ever drop me back to the initial router I established telnet to.

Sometimes you have to hit ctrl-shift-6 a few times but it never drops me back and I do this many times a day.

paul.matthews Thu, 09/13/2007 - 06:01

Disagree all you want, but when you are multi hopped through routers, the first router to "see" the sequence you type is the first router you are connected to. As the effect of the sequence is to pull you back from the telnet those keystrokes cannot get to next hop sessions.

robert.ward Thu, 09/13/2007 - 06:25

I just tried it to be sure and telneted (hop by hop)across five routers started a trace route running to a distant site, did ctrl-shift-6 and it broke out to the edge router I was running the trace from. I didnt break out to the intial router so I am confused why you believe that.

I wonder if there are configs that can influence that action and returns you to the initial router.

If your in a position to try it, I would be interested to know if you find that it throws you back to the initial router as mine do not.

robert.ward Thu, 09/13/2007 - 06:43

Its odd we get differing results and I have worked for several telcos and not seen it any different. My telnet client is putty so not sure if it does it slightly different.

example below of me telnet'd across some routers on their loopbacks and doing a trace then breaking out. names and ip's have been hidden. I dropped out using the ctrl-shift-6 and then had to exit from each one.

www_xxxxx>trace x.x.1.1

Type escape sequence to abort.

Tracing the route to x.x.1.1

1 x.x.43.9 12 msec 12 msec 12 msec

2 x.x.43.1 24 msec 24 msec 24 msec

3 * * *

4 * * *

5 * * *

6 * * *






[Connection to x.x.43.31 closed by foreign host]


[Connection to x.x.43.30 closed by foreign host]


I would be interested to know how many people see similar results as there may be config or IOS differences that react in a different manner.

willemvwyk Tue, 09/18/2007 - 01:52


My apologies for the late reply. I telnet to router1 which is a 3640 with 12.0(22) IOS. From there I connect to router2 another 3640. Router2 has 12.2(11)T6 IOS on. When I start a trace from router2 and hit ctrl-shift-6 x I go back to router1. Once I type in resume and hit enter I am back on router2 with the trace still going. I would be very interested to see how Paul is not being returned to router1 when he does the break sequence on the router?



Paolo Bevilacqua Tue, 09/18/2007 - 02:00

You are always returned to the first router you were connected. One technique is that you change the escape sequence to something else under config line (escape-character).

Kevin Dorrell Tue, 09/18/2007 - 02:14

If we are taking a poll on this, mine goes back to the first one in the chain. I set up three hops through 4500 switches running 12.2(25)EWA2, then started a trace, and interrupted with Ctrl-Shift-6 X. This took me back to the first switch in the chain.

However, Robert is very insistant that he only interrupts one router, and I see no reason to doubt his word. In fact, I wish all systems would behave as his does. So, Robert, what have you got that we do not?

Kevin Dorrell


Richard Burts Tue, 09/18/2007 - 03:07

I believe that we are talking about subtly different things. The essence of the difference is whether there is an x at the end of the sequence. If you type an x it will suspend the telnet and put you back on one of the routers in the telnet chain. If you do not type an x it will not suspend and will break the traceroute. I agree that you may need to type the SHFT-CTL-6 several times but in my experience it does stop the traceroute (and without the x will not suspend back to the source router).

On the subject of which router it goes back to I would suggest an experiment to explore this. Set up a chain of telnet from router to router for 3 or 4 routers. Then type the SHFT-CTL-6 two times (2 times) before you type the x. Then tell me what router you wind up on. I believe that it will not be the original source router.



robert.ward Tue, 09/18/2007 - 07:19

I get the traceroute to break out and I stop on the end router I was doing the traceroute from. I am not returned back to previous routers I have hopped along until I exit each router in turn. It is strange other people get dumped back to the first router they are on.


Kevin Dorrell Tue, 09/18/2007 - 07:29

OK, I got it now. I set up a chain of three telnets and tried various things.

Ctrl-shift-6 then x will take you back to the first router in the chain.

Ctrl-shift-6 twice will abort the current trace without terminating any telnet session. BUT, the two Ctrl-Shift-6 must be done withing the time of a single trace probe. (Per probe, NOT per line of three probes.)

Interestingly, when I did two ctrl-shift-6, then x, I got dumped on the middle router in the chain. Do we see a pattern here.

That is basically what Rick was saying. Does anyone agree?

I then jumped to a fourth router. Now I needed 3 ctrl-shift-6 to abort the trace. I begin to see some patterns, but there is some funny timing involved.

I think Willem gave up and went home!

Kevin Dorrell


Kevin Dorrell Tue, 09/18/2007 - 07:42

OK, how's this for a model.

The first ctrl-shift-6 goes to the first router, which waits for a command: X to abort telnet or another character to continue.

If it receives another ctrl-shift-6 within a second then it does not abort, but passes it to the next router. An x will now abort the telnet at the second router.

Another ctrl-shift-6 within 1 second will get passed to the third router. An x will now abort the telnet at the third router.

Etc. etc.

And if you put in a series of ctrl-shift-6 at intervals of less than a second for the number of hops in your chain, then you get to abort the trace.


Kevin Dorrell


robert.ward Tue, 09/18/2007 - 07:28

ok ths is interesting.

on a router four hops away I get the following.

ctrl-shift-6-x once or twice it drops to the first router and suspends the end one doing trace until I drop back to it.


doing ctrl-shift-6 several times as I normally do will break out onto the current last telneted router connection and stays there.

both options work and seem to do two different things. Once ends and the other suspends and drops back to the initial router.



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