[j-nsp] MPLS labels on a traceroute across an IXP?

[Gavin Henry]

>> on a traceroute before (being new to MPLS too :-) ) and presume it's
>> showing because of the junos traceroute, whereas I don't see it from
>> the same
>> network on a linux box?
>
> Yes. You need to use the -e flag if you want it to show on a Linux traceroute.

Ah, cool. I should have man traceroute / MPLS :-)

So, running that again with -e didn't show the same thing as the junos one.

traceroute -e on linux:

 3  lonap-gw2.plus.net (193.203.5.155)  13.001 ms  12.993 ms  13.022 ms
 4  te-9-1.ptw-gw02.plus.net (212.159.1.19) <MPLS:L=1274,E=0,S=1,T=1>
13.512 ms  13.607 ms  13.795 ms
 5  link8-central10.ptw-ag01.plus.net (84.93.248.15)  17.695 ms
18.456 ms  19.484 ms

and on JUNOS Software Release [10.4R11.4] (Export edition):

 3  lonap-gw2.plus.net (193.203.5.155)  15.569 ms  22.826 ms  22.150 ms
 4  te-9-1.ptw-gw02.plus.net (212.159.1.19)  186.604 ms  197.964 ms  202.049 ms
     MPLS Label=9814 CoS=0 TTL=1 S=1
 5  link6-central10.ptw-ag01.plus.net (84.93.248.11)  19.876 ms
18.910 ms  18.918 ms

The label is different, but always the same on each version. I presume
that's because the linux one,
even though using the same gateway as the junos router, is going on to
link8 and the junos one on to
link6.

>> My other question is should this actually show and not be remove by
>> the egress LER?
>
> Without ICMP extensions, when an LSR receives an undeliverable
> MPLS-encapsulated datagram, it strips the MPLS label stack and punts
> that IP datagram to the error processing module, which then generates
> the, e.g., Time Exceeded ICMP.
>
> With ICMP extensions, the LSR instead provides the MPLS label stack to
> the error processing module, which appends that information to the
> ICMP using a "MPLS Label Stack Object".
>
> So, as long as the router received the datagram as MPLS, it can/will
> send back the ICMP with a MPLS Label Stack Object. This applies to an
> LER.

Thanks Darius, much appreciated. That's probably covered in my
MPLS-Enabled Applications book
that I've just started...maybe not.

Thanks,

Gavin.

-- 
Kind Regards,

Gavin Henry.
Managing Director.

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