The MPLS-OPS Archive

[Yves@Fauser.de]

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

The external network is learned via static (E2) from PE2. Since the prefix is learned via static, there is no PHP on the P device.
Now if we do a traceroute from the Test WS (Global RT) to a host in the external network, we get an answer from every hop. What I don't understand is what we see in the Sniffer trace.
In the trace the first thing we see is an "ICMP time exceeded" sourced by PE2 and send to the FireWall with the destination "Test WS". I understand this, since PE2 is sending the "TTL exceeded" message up the LSP to the FireWall, which sends it back, as described in RFCs and in various books. 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.

Thanks, Yves