MPLS implicit Null vs Explicit Null

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.