[nsp] traceroute question

[Iva Cabric]

On Mon, Feb 17, 2003 at 09:50:21PM +0100, Yves at Fauser.de wrote:
> Hi Folks,
> 
> I have a question which might be an implementation specific issue, or an 
> misunderstanding of some concepts from my part. here's the setup : - We 
> have a small MPLS Network with 25 PEs and 5 Ps. All of them are Cisco7600 
> (Catalyst6500) with 4-GE-WAN OSMs. We are still in the migration phase, so 
> our Global-Routing-Table is still used for a bunch of prefixes. We use 
> OSPF in a single area as IGP and TDP.
> 
> Test WS  ---- PE1  -----  P  ----- PE2 ----|Sniffer|----- FireWall ----- 
> External Network
> 
[...]
> 
> What I don't get is why we don't see the "ICMP Time Exceeded" send from 
> the P device. If I understood the concepts right, we should see this in 
> the trace, but we don't. Unfortunatly I didn't get the chance to do a 
> Sniffer trace between the P and PE2 (lack of a Gigabit-Sniffer). Since we 
> get a response from the P device in the traceroute output, either PE2 or P 
> sends the "ICMP TTL Exceed" from the P device back to the workstation.
> 
> So my main question is : - Should we see a "TTL Exceeded" with a source 
> address of the P device and the workstation as destination in the Sniffer 
> trace, or did I miss something in the concept.

It depends on configuration, if you have "no tag-switching ip propagate-ttl"
on P it won't report itself as hop, but with "tag-switching ip propagate-ttl"
you will see it in traceroute. See following document for more info:
<URL:http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t3/rtr_13t.htm#1067919>