The MPLS WG Archive

[Eric Rosen <erosen@cisco.com>]

1) Section 3 says: 
   "With this restriction in place, one should not distribute, to a
   particular label distribution peer, a binding of Explicit NULL to a
   particular FEC, unless the following condition (call it "Condition
   L") holds:  all MPLS packets received by that peer with an incoming
   label corresponding to that FEC contain only a single label stack
   entry.  If Explicit NULL is bound to the FEC, but Condition L doesn't
   hold, the peer is being requested to create illegal packets.  None of
   the MPLS specifications say what the peer is actually supposed to do
   in this case.  This situation is made more troublesome by the facts
   that, in practice, Condition L rarely holds, and it is not possible
   in general to determine whether it holds or not."

Shahram> If an egress router does not distribute the explicit null label for
Shahram> a set of FEC  elements, when it is not the egress  LER for at least
Shahram> one of those FEC elements, then condition L always holds.

Shahram> Am I missing something?

Consider a  packet of FEC  F traveling from  LSR A through  LSR B to  LSR C.
Suppose that C distributed to B a label, L1, for F, and B distributes to A a
label, L2, for F.  

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.