[c-nsp] IP LFA in ring topology

[Adam Vitkovsky]

> I can understand how it works for INET, 
>but I can't figure out how it would work for VPN.

I believe it has to work the same as with mpls-te frr
Where the merge point router has to tell you what label he'll use to swap
and forward towards the final PE
So that you can than impose a label stack that would get you to the merge
point router as a tunnel endpoint
and when the top label from this stack is popped because of php
the merge point router gets a packet with a label he provided and will swap
it with the one towards the final PE
and the whole time at the bottom of the stack there can be other labels like
vpn or pw label

adam