classic traceroute thru MPLS VPN

Apr 20, 2009 2 Replies

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Piotr 17 years ago

Gents,

I'm running traceroute from local CE router to remote CE router thru ISP MPLS VPN in the same VPN. I assume that my ISP has 'no mpls ttl propagation'.

1 10.0.5.133 0 msec 0 msec 4 msec 2 10.0.1.153 [AS 65000] [MPLS: Label 1420 Exp 0] 8 msec 4 msec 0 msec 3 10.0.1.154 [AS 65000] 8 msec * 8 msec

10.0.1.153 - ISP

10.0.1.154 - Serial0 - remote CE router

Question: Why in the traceroute output the second hop is the IP address (10.0.1.153) of the outgoing interface which interconnects my remote CE to ISP (10.0.1.152/30 subnet)?

I would expect to see some interface faced towards local CE from MPLS cloud or none. What is the theory behind it that the ISP router is sourcing the ICMP ttl exceeded message with mentioned IP address? Does the ISP edge PE router some additional checking of the IP TTL after poping labels?

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P

Piotr 17 years ago

Gents,

I'm running traceroute from local CE router to remote CE router thru ISP MPLS VPN in the same VPN. I assume that my ISP has 'no mpls ttl propagation'.

1 10.0.5.133 0 msec 0 msec 4 msec 2 10.0.1.153 [AS 65000] [MPLS: Label 1420 Exp 0] 8 msec 4 msec 0 msec 3 10.0.1.154 [AS 65000] 8 msec * 8 msec

10.0.1.153 - ISP

10.0.1.154 - Serial0 - remote CE router

Question: Why in the traceroute output the second hop is the IP address (10.0.1.153) of the outgoing interface which interconnects my remote CE to ISP (10.0.1.152/30 subnet)?

I would expect to see some interface faced towards local CE from MPLS cloud. What is the theory behind it that the ISP router is sourcing the ICMP ttl exceeded message with mentioned IP address?

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P

Piotr 17 years ago

Here is the answer for this question. The Cisco IOS does additional check when packet is leaving egress PE for TTL 1/0 -

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