traceroute behavior in MPLS VPN network

Question:

I'm trying to WS-C3750V2-48PS-S understand traceroute behaviour in an MPLS VPN network. Below is the traceroute from our management station (Linux machine) to the CE router Loopback address. From what I understand, if TTL expires on a P router (hop 15-18), using ICMP extension, P then will use the Label information received and attached to ICMP TTL expired message packet and forwards it along the LSP since P does not know how to reach the source. P routers will use IP address on the receiving interface as source for ICMP TTL expired message. The confusion is when TTL expires on PE router.

I can see it is using the IP address on the vrf interface instead the IP of the receiving Interface (WAN interface). I've googled but cannot find a satisfying answer. Can someone please explain? Thank alot.

~$ traceroute -n -f 15 10.0.17.136

traceroute to 10.0.17.136 (10.0.17.136), 30 hops max, 38 byte packets

15  172.30.253.49  310.019 ms  310.464 ms  310.091 ms

     MPLS Label=397 CoS=5 TTL=1 S=0

     MPLS Label=1435 CoS=0 TTL=1 S=0

16  131.228.129.53  309.899 ms  310.079 ms  315.642 ms

     MPLS Label=541 CoS=5 TTL=1 S=0

     MPLS Label=1435 CoS=0 TTL=2 S=0

17  131.228.129.130  309.527 ms  309.825 ms  313.037 ms

     MPLS Label=1898 CoS=5 TTL=1 S=0

     MPLS Label=1435 CoS=0 TTL=3 S=0

18  172.28.87.10  309.917 ms  310.807 ms  309.786 ms

     MPLS Label=136 CoS=5 TTL=1 S=0

     MPLS Label=1435 CoS=0 TTL=4 S=0

19  10.58.24.253  309.183 ms  310.194 ms  309.008 ms

     MPLS Label=1435 CoS=5 TTL=1 S=0

20  10.58.24.254  308.081 ms *  310.713 ms

Answer:

First check this article : http://blog.ine.com/2008/11/24/mpls-ping-and-traceroute/

it's very detailed.

With regards to your question:

"The confusion is when TTL expires on PE router.

I can see it is using the IP address on the vrf interface instead the IP of the receiving Interface (WAN interface)."

Yes, it's expected that remote PE will reply with the ip of vrf interface, P will reply with the interface ip facing the WS-C3750X-12S-S Price source.

For more Cisco Switch news about Price ans Specification, you can click here.

http://www.3anetwork.com/cisco-ws-c3750x-12s-s-price_p111.html