The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

George Thomas wrote:
> 
> If there is no LSP associated with the destination, (say for e.g.
> unicast/muticast/broadcast packet), the packet can be sent as
> native IP packet without NULL label encapsulation in a MPLS domain
> or it can be discarded, based on the design of the router. This is
> my understanding from your response.

Correct.

> Then i would like to know when NULL label encapsulation is to be
> used?

A router that isn't capable of popping a label stack may be required to
do so.  In this situation, it can swap to NULL.  When this happens, the
next-hop router is expected to pop off that NULL label prior to
processing the packet.

> My initial understanding was: In a MPLS domain, for IP packets
> which cannot be associated with a LSP, those packets should be
> sent to the IP next hop with NULL label encapsulation in a LAN
> media and it should not be sent as native IP packet.

NULL labels are a workaround to get around some kinds of hardware
restrictions.  I don't think they were ever meant to be the primary
means of forwarding unlabeled traffic.

-- David