In some cases a service provider may wish to carry a CoS value within the shim label of an LSP, and apply QoS policies that are either independent of the CoS value contained in the packet headers below the shim label, or to ensure QoS policies are applied to the end to end LSP.
Explicit null is an optional configuration that enables this requirement by allowing an MPLS LSR (that resides one hop before the final hop in an LSP) to maintain the application of an MPLS shim label to packets that are destined towards the egress LSR.
This overrides the default Penulitmate Hop Popping (PHP) behaviour (or implicit null), which signals a label value of 3 to the penultimate router, instructing it to not push a shim label when forward to the egress LSR.
When explicit null is enabled a label value of 0 is signalled to the penultimate LSR, which instructs the penulitate LSR to insert a shim label when forwarding packets to the egress LSR.
Configuration Step 1
Configuration Step 2
Under the LDP heirary apply the same command again.