[c-nsp] DFZ-in-a-VRF: ASR1k per-ce label TTL troubles

[Lukas Tribus]

> Hi Lukas, 
>  
> That is an interesting "feature" 
> I would expect the same behavior in both cases i.e. with per-vrf label  
> the router pops the VPN label and founds a packet with TTL 1 decreases  
> and sends to CE. 
> And in case of the per-NH label the packet should be switched out the  
> egress interface based on the VPN label and again the TTL in IP header  
> is decreased and packet sent to CE that generates the ttl expired msg  
> upon receipt.

In the meantime I have been able to confirm that both per-vrf *and*
the default per-prefix label allocation mode work fine, its just the (pretty
recent) per-ce mode that seems to be affected by this behavior.

Thats surprising because per-prefix and per-ce label mode are both
theoretically the same from a data-plane perspective, the label always
points to a layer 2 rewrite information, so there is no IP lookup involved.
It is also exactly the same thing from an ingress LER and LSR point-of-view.

I think the code path when label switching is different in the per-NH and
per-prefix case, that would explain the difference in behavior (especially
because per-ce label allocation mode is a recent feature).

Anyway, TAC can reproduce this, the BU has been pinged and I don't
currently see how this can be misconstrued as feature.


Lukas