[c-nsp] traceroutes via mpls network - works for on-net but not for off-net (def rt)

[Christian Meutes]

On 2014-09-04 19:16, Aaron wrote:
> In my network traceroute works fine for on-net (known) subnets. I can 
> see
> the mpls lsr P hops.
>
>
>
> But when I traceroute to internet destinations off-net (unknown) 
> subnets and
> my packets follow default routing, I do not see my mpls lsr P hops.
>
>
>
> What is the deal with traceroute being broken when following the 
> default
> route ?

Just a guess:

Remember that the ICMP ttl-exceeded packet gets switched to the LSPs 
tail-
end LER/PE where IP processing can happen, but for a learned 
default-route
will most probably not occur and instead packets get MPLS-switched to 
the
default-routes l2adjacency on your ISP-facing LER/PE directly (without
consulting the VRF). Hence my guess is that your ISPs router doesn't 
want
to route the ttl-exceeded packets back to you (maybe URPF ingress -> 
you
have private linknetworks sourcing the ICMP-ttl's?).

Cheers
Chris