Hi Experts,
I'm trying to 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
Regards,
Sam
