[c-nsp] Newbie traceroute question

sthaug at nethelp.no sthaug at nethelp.no
Wed Feb 10 01:47:28 EST 2016

> On hop 2, 6, 7 & 8 suddenly the traceroute output *includes *MPLS data
> (label & exp).  I understand how traceroute works (am very familiar with
> Richard's excellent preso), but I still haven't seen an answer to my
> question.  Gert's was possibly the closest with "Nothing special in IOS
> traceroute, but more "stupidity in general unix traceroute" - when
> sending a TTL exceed ICMP packet, the LSR copies the label stack into
> the ICMP packet, and traceroute *could* handle this."

And Cisco routers by default decodes the MPLS label stack that has
been copied into the ICMP packet - *even if the router itself is not
configured for MPLS*.

There's really no magic here.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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