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Community Member

traceroute

Hi,

If I am doing an extended traceroute with a source address not local to the router in which I am doing the traceroute...

Fine ,the first ICMP packet will go to the first hop (primary path to the destination address in my router's routing table)to reach the destination address , and it gets dropped there...and that router needs to send an ICMP type 11 code 3...It will look in to the routing table to see the reachability of the traceroute "source address" ,which is his destination address..And if that address is reachable via another hop (the prefered path in that router's routing table) ,it will send the ICMP type 11 code 3 , thru that hop...so my question is which path will be taken by the next ICMP(TTL2) sent by the originating router...the path taken by the first ICMP ,or the reverse path in which the ICMP type11 had came back to the host

thanks in advance,

Thomas.

4 REPLIES
Hall of Fame Super Gold

Re: traceroute

Thomas

The answer to your question is that there is not anything about the ICMP type 11 and the path that it uses that will change the routing logic of the router doing the traceroute. Assuming that there has not been anything that changed the forwarding table of the router it will make exactly the same routing decision for the next traceroute packet. If there is a single route to the destination it will use the same path. If there are two equal cost routes to the destination it will use the second path.

What you are asking about is assymetric routing where the return packet does not take the same path as the outbound packet. While it is usually not desirable there is not anything inherently broken about assymetric routing (until you get into things like stateful inspection of traffic).

HTH

Rick

Community Member

Re: traceroute

Hi Rick,

thanks for your reply..

<<>>

So if there are 2 equal cost paths ,the first ICMP TTL1 goes thru the first path ,second ICMP TTL2 thru the second path ,third ICMP TTL3,thru the first path and so on..?

is this what you meant or whether I got you wrong..?

Thanks

Thomas.

Hall of Fame Super Gold

Re: traceroute

Thomas

That is what I meant. Actually if we want to be precise there are 3 probes at TTL of 1, 3 probes at TTL of 2, etc. The router will send first packet on path 1, second packet on path 2, third packet on path 1 (end of TTL 1), fourth packet on path 2 (beginning of TTL 2), fifth packet on path 1, sixth packet on path 2 (end of TTL 2) and so on.

HTH

Rick

Community Member

Re: traceroute and how to use SH IP ROUTE along with it to know that what actual path traffic is taking

Hi Ev eryone ,

 

Can anyone help me out as my client is having issue in reaching a Application server . I have asked them for the PING / TRACE ROUTE results . Can anyone help me how use the Traceroute information ( HOPS) , along with the use of SH IP ROUTE , to know what exact path the traffic is taking to determine the issue . i donot know how to check the check the SH IP ROUTE and determine that what path it's taking and if going Outside to WAN . Generally people dol sh ip route by taking any HOP IP from traceroute results ( that client have shared) and they determine it by again and again doing sh ip route ( from taking the IP address showing in the output of sh ip route in the line starting with Asterick * ), and then they continue logging several devices and they finally determine the issue . CAN anyone PLEASE EXPLAIN ME WITH AN EXAMPLE .

 

THANKS !!! CHEERS !!

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