12-18-2012 08:05 PM - edited 03-04-2019 06:26 PM
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
Solved! Go to Solution.
12-20-2012 12:57 AM
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 source.
12-20-2012 12:57 AM
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 source.
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