Solved: Traceroute o/p on cisco router

anasubra_2 · ‎07-16-2007

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

Richard Burts · ‎07-16-2007

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

HTH

Rick

View solution in original post

Harold Ritter · ‎07-16-2007

The default in IOS is to rate limit the "iCMP destination unreachable" messages to two per second.

Please refer to the IOS documentation for more information on this default behavior:

http://www.cisco.com/en/US/products/ps6441/products_command_reference_chapter09186a00804a8b24.html#wp1011819

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

anasubra_2 · ‎07-16-2007

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

Richard Burts · ‎07-16-2007

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

HTH

Rick

Harold Ritter · ‎07-16-2007

Rick,

As always, you are absolutely correct.

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Richard Burts · ‎07-16-2007

Harold

Thanks for the confirmation.

HTH

Rick

HTH

Rick

anasubra_2 · ‎07-17-2007

Hi Rick,

Got it, Thank you very much ...

Regards

Anantha Subramanian Natarajan

Richard Burts · ‎07-17-2007

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

HTH

Rick

anasubra_2 · ‎07-17-2007

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 · ‎07-17-2007

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

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Richard Burts · ‎07-17-2007

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

HTH

Rick

rakesh.shah · ‎07-17-2007

got it. Thanks for this input.

regards

Rakesh

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