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Traceroute Question

Posted on 2009-04-09
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Last Modified: 2012-05-06
I do a trace route to an address on a switch called S1

From R1 the traceroute shows R2 - R3 - S1 (works)
From R2 the traceroute shows R3 and then it gets stuck
From R3 the traceroute shows S1  (works)

Same address:
Ping from R1: OK!
Ping from R2: fails
Ping from R3: OK!

I'm a networking newbie and I don't understand how a traceroute that goes thru R2 works but if initiated from R2 it doesn't work.

What could account for that behavior?  
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Question by:MajorBigDeal
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by:ionut_mir
ID: 24113653
It seems like the switch doesn't "know" how to reach the IP address of R2.
The trace goes through R2 but with R1's source IP address.

Check routing table (if it has) on S1. Or put a route so that any packet generated by the S1 to "know" how to get to R2.
If you have a default gateway on S1 pointing to R3, than check R3's routing table.
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by:MajorBigDeal
ID: 24114110
Sorry, I'm really confused.  Do you mean that R2 doesn't know how to reach the switch? Or maybe I'm missing a basic concept.  

I'm starting at R1 then it goes to R2 and then to R3 and then to the switch (S1).  So doesn't that mean that R2 must know how to get to S1?

But if I start at R2, it doesn't know.
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Expert Comment

by:ionut_mir
ID: 24114135
When you are starting the trace at R1, the packet leaves the R1 with a source IP address which never changes (unless a nat, but I doubt it). It reaches R2 (this router responds with an echo reply), and then the packet travels away to R3 router - the source IP address is the same (R1's address). When it receives the packet S1 knows how to get to R1.

But when you are starting at R2 , on the way back the switch doesn't know how to get to R2.

Try to follow, step-by-step, the packet on his way to the switch, and the way back as well. Check if at any point the device has an way to the destination address.

I think that the switch doesn't have a route to R2. I hope I am not mistaking!
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by:MajorBigDeal
ID: 24114325
Ah, thank you that makes more sense now.

So, if I am on S1 I should not be able to ping R2, is that true?

I can't check for another 9 hours but the reason I ask is that I think I am able to ping R2 from S1.
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Expert Comment

by:ionut_mir
ID: 24114616
This is my guess, yes.

If the switch can ping R2 then you should check a little more carefully the entire path of access-lists or any other causes that could prevent icmp packets to get to S1.

Good luck!
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by:MajorBigDeal
ID: 24118853
RIght, I was just able to test and S1 can ping R2.  

So what your are saying is that an access-list could prevent a ping from originating on R2 while allowing it to traverse R2? I'll look again keeping that in mind.  Please let me know if that is not what you are saying.
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by:MajorBigDeal
ID: 24119147
I don;t see any access-lists applied to any interfaces.  There is a vlan though.
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ionut_mir earned 1000 total points
ID: 24121528
It is a little bit odd what is happening... you can't ping S1 from R2 but you can ping R2 from S1.
If it is possible, try to post some configs that you consider relevant, and a little drawing of your network, maybe we are missing something here...
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by:MajorBigDeal
ID: 24181698
Yes, I'm sure we are missing something here and I appreciate your willingness to help.  This has now become the problem of someone much more experienced in networking than I and he is also having difficulty. Anyway, I'm moving on.
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