Traceroute o/p on cisco router

Answered Question
Jul 16th, 2007
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Hi,


When I traceroute to a destination ip address in the network from the router, the last hop to reach the destination has one timed out problem as below


router#traceroute <dest-ip> address


Type escape sequence to abort.

Tracing the route to <dest-ip> address


1 <next-hop> 4 msec * 0 msec


I am just curious and would like to understand the reason for the 2nd probe getting timed out. I am not finding bad links and so wondering why that probe should be dropped. This characteristics, I am seeing in most of the networks and its particularly for the last hop when traced from the router.


Thanks


Regards

Anantha Subramanian Natarajan

Correct Answer by Richard Burts about 9 years 8 months ago

Anantha


I believe that the difference is that the response from the intermediate hop is not destination unreachable but is Time To Live Expired. The IOS does not throttle these messages like it does the destination unreachable messages.


HTH


Rick

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anasubra_2 Mon, 07/16/2007 - 12:27
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Thank you very much and its really good to know .............


But I am just curious how the intermediate hop doesn't get timed out as below


router#traceroute


Type escape sequence to abort.

Tracing the route to


1 < first-hop> 12 msec 12 msec 16 msec

2 < second-hop> 12 msec * 12 msec


Is that something I am missing .....


Once again, thanks for this answer.


Regards

Anantha Subramanian Natarajan



Correct Answer
Richard Burts Mon, 07/16/2007 - 12:43
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Anantha


I believe that the difference is that the response from the intermediate hop is not destination unreachable but is Time To Live Expired. The IOS does not throttle these messages like it does the destination unreachable messages.


HTH


Rick

Harold Ritter Mon, 07/16/2007 - 15:42
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Rick,


As always, you are absolutely correct.


Regards,

Richard Burts Mon, 07/16/2007 - 18:30
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Harold


Thanks for the confirmation.


HTH


Rick

anasubra_2 Tue, 07/17/2007 - 08:32
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Hi Rick,


Got it, Thank you very much ...


Regards

Anantha Subramanian Natarajan

Richard Burts Tue, 07/17/2007 - 10:39
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Anantha


I am glad that we were able to help you resolve your questions. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read an answer that did resolve the question. I encourage you to continue your participation in the forum.


HTH


Rick

anasubra_2 Tue, 07/17/2007 - 14:03
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Hi Rick,


You people deserve the rating and I always liked this forum because it gives me answers :) ..I mean understandable answers.


Once again thanks and bye


Regards

Anantha Subramanian Natarajan

rakesh.shah Tue, 07/17/2007 - 10:57
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Rick,

my 2 cents-

There could be one more reason for this behaviour - the intermideate nodes has to make only routing/switching function, while the destination node has to reply to the service (in this case icmp/trace). ping/tracert are categorised as low priority by default. So depending upon the free/busy status of end node, the destination node may able to reply tracert request in timely manner or may not.

regards

Rakesh

=====

Richard Burts Tue, 07/17/2007 - 11:16
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Rakesh


Actually when each intermediate node receives the traceroute probe packet, decrements the TTL, and if it gets to zero, then they must discard the packet and they must generate the ICMP error message about time exceeded. So each intermediate node must generate error messages similar to the error generated by the destination router. So the processing load is pretty much the same in the intermediate nodes as it is in the destination.


HTH


Rick

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