The MPLS WG Archive

[Shahram Davari <Shahram_Davari@pmc-sierra.com>]

Hi Eric,

>
>Condition L states that if B receives  an MPLS packet from A, 
>and if the top
>incoming label on that packet is L2, then the packet has only 
>a single label
>stack entry.
>
>Whether  this condition holds  depends on  what has  happened 
>to  the packet
>before it gets to B. 
>
>The problem arises  if the condition doesn't hold, but  L1 is 
>explicit null.

If C has distributed an Explicit NUll Label (L1) for FEC F (F presumably is a set of IP prefixes)
to B, then it means that C is the egress LER for that FEC, meaning that C wants to forward packets destined to FEC F based on their IP addresses.

If B has distributed a label (L2) for FEC F to A, then it is impossible for A to send a packet with more than 1 label (label L2) to B. Because if A does that, then A effectively wants the second label to terminate somewhere downstream of C, in which case C knows that it is not the
egress LER for FEC F and therefore it should not distribute the explicit null label for FEC F to B.

Am I missing something?

-Shahram