The implicit null label option allows a router egress LER to receive MPLS packets from the previous hop without the outer LSP label. The operation of the previous hop is referred to as penultimate hop popping (PHP).
This option is signaled by the egress LER to the previous hop during the LSP signaling with RSVP control protocol. In addition, the egress LER can be configured to receive MPLS packet with the implicit null label on a static LSP.
The user can configure your router to signal the implicit null label value over all RSVP interfaces and for all RSVP LSPs for which this node is the egress LER using the implicit-null-label command in the config>router>rsvp context.
The user must shut down RSVP before being able to change the implicit null configuration option.
The user can also override the RSVP level configuration for a specific RSVP interface:
config>router>rsvp>if>implicit-null-label {enable | disable}
All LSPs for which this node is the egress LER and for which the path message is received from the previous hop node over this RSVP interface signal the implicit null label. This means that if the egress LER is also the merge-point (MP) node, then the incoming interface for the path refresh message over the bypass dictates if the packet uses the implicit null label or not; the same applies to a 1-to-1 detour LSP.
By default, an RSVP interface inherits the RSVP level configuration. The user must shut down the RSVP interface before being able to change the implicit null configuration option.
The RSVP interface must be shutdown regardless of whether the new value for the interface is the same or different than the one it is currently using.
The egress LER does not signal the implicit null label value on P2MP RSVP LSPs. However, the PHP node can honor a Resv message with the label value set to the implicit null value when the egress LER is a third party implementation.
The implicit null label option is also supported on a static label LSP. The following commands can be used to cause the node to push or to swap to an implicit null label on the MPLS packet:
config>router>mpls>static-lsp>push implicit-null-label nexthop ip-address
config>router>mpls>if>label-map>swap implicit-null-label nexthop ip-address