Traceroute in MPLS

Hi All,

I am trying to get an understanding of the way traceroute works in MPLS.
This is what I understood by reading Cisco material on traceroute in MPLS:
The topology R1 through R3 is MPLS enabled routers.

C1 ------ R1 ------ R2 ------ R3 ------ C2

1. Traceroute packet sent by C1 with TTL 1 is dropped by R1.
2. C1 sends another packet with TTL 2, R1 decrements TTL by 1 and forward to
R2
3. R2 decrements TTL to 0 in MPLS header and drops the packets. In order to
send the ICMP error message, the R2 needs to find out the source address. R2
derives source address from IP header, but it doesn't have route back to C1.
The cisco document says "Router B forwards the ICMP messages along the same
label switched path (LSP) that the dropped packet was traveling (in the
direction toward router C). At the end of the LSP, the label is removed and
the ICMP messages are forwarded according to the destination address in the
IP header (toward Router C1)".

Does it mean, the packet has to reach the destination, so it can derive the
source address of C1. If that's the case, why do C1 again create packet with
TTL 3 and TTL 4 to receive the port unreachable command. Confused!!

Also if I disable *mpls ip propagate-ttl,* it says the P routers will not be
shown in traceroute command. As the R1 wil decrement the TTL and assign TTL
of 255, R2 will decrement to 254 and forwards to router C, where it will
copy TTL to IP header and forwards to C2.

Any suggestion to clarify my doubts will be highly appreciated.

Thanks,

Bilal Hansrod

Blogs and organic groups at http://www.ccie.net
Received on Sun Apr 24 2011 - 20:45:31 ART