[c-nsp] Newbie traceroute question

Gert Doering gert at greenie.muc.de
Tue Feb 9 13:52:47 EST 2016


On Tue, Feb 09, 2016 at 08:26:46PM +0200, Hank Nussbacher wrote:
> When doing a traceroute from a Cisco IOS, what is being done under the
> hood so that sometimes I am able to see MPLS hops from within the
> traceroute?

As far as I understand, it's really 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 (part of it?  all?)
into the ICMP packet, and traceroute *could* handle this.

When I traceroute from my FreeBSD machine to one of the routers @work,
traceroute doesn't show MPLS info, but tcpdump on the BSD host does:

traceroute to (changed), from (also changed),
eliciting a reply from (also changed):

19:41:47.221438 IP (tos 0xc0, ttl 249, id 60178, offset 0, flags [none], proto ICMP (1), length 168) > ICMP time exceeded in-transit, length 148
        IP (tos 0x0, ttl 1, id 55010, offset 0, flags [none], proto UDP (17), length 52) > UDP, length 24
        MPLS extension v2, checksum 0x6dc6 (correct), length 12
          MPLS Stack Entry Object (1), Class-Type: 1, length 8
            label 759, exp 0, [S], ttl 1

... traceroute'ing from a cisco along the same path, I get...

  2 Router ( [MPLS: Label 759 Exp 0] 16 msec 8 msec 204 msec

Now, my *Linux* traceroute has a "-e" flag that will actually display
MPLS data (didn't know that, good I looked :-) )

 3  Router ( <MPLS:L=759,E=0,S=1,T=1>  92.362 ms  92.385 ms  92.364 ms

It also claims the MPLS ICMP extention is RFC4950.


USENET is *not* the non-clickable part of WWW!
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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