Hi Zakor,
When you configure MPLS explicit-null labels, label 0 is advertised to the egress router of an LSP. When label 0 is advertised, the egress router (instead of the penultimate router) removes the label. Ultimate-hop popping ensures that any packets traversing an MPLS network include a label
We can say the implicit-null label is used by default and works well if the network has not particular QoS requirement, or if the QoS is end to end between the CPEs. On the other hand, the explicit-null label is used when the MPLS core network needs to know the QoS values.
Hope this helps
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