Implicit-Null vs. Explicit-Null label
The purpose of null label
assignment on Egress PE is to avoid unnecessary MPLS encapsulation on the P LSR
connected to the Egress PE, and also to avoid unnecessary MPLS decapsulation on
the Egress PE.
Both implicit and explicit null labels are generated by last hop router to
its neighbors.
Implicit null is by default which means penultimate router should only send
IP packet thus it pops the label (popping the label known as PHP and this is
done to reduce the load on last hop router). The one disadvantage in implicit
null approach is if the network is configured for QoS based on MPLS EXP bits,
then QoS is lost between penultimate router and last hop router.
In this case, we can make use of Explicit null which means penultimate hop
router does not pop the label. It sends with label value of 0 but with other
fileds including EXP bits intact. This way QoS treatment is preserved between
penultimate router and last hop router. Explicit null should be configured
manually in last hop router.
Configure "mpls ldp explicit-null" in the last hop router.
Options of specifying to which FEC explicit null are generated and to which LDP
neighbors it is advertised can be controlled with keywords "for" and
"to".
PE7 (config)#mpls ldp explicit-null ?
for
Access-list specifying controls on destination prefixes
to
Access-list specifying controls on LDP peers
