Exam JN0-361 topic 1 question 19 discussion

C. Configure explicit-null on all MPLS-enabled devices. I think "C" is the correct answer.

we should configure ultimate hop pop only on ingress router

Its A because, by default egress LSR signals to the PHP router for implicit-null label so the PHP LSR can send an ip packet instead of labeled packet. But we want to be able to copy the qos information on the egress LSR so it needs to signal explicit-null label to PHP LSR: https://www.juniper.net/documentation/en_US/junos/topics/topic-map/lsp-labels.html

I think the answer is C: 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. https://forums.juniper.net/t5/Routing/MPLS-explicit-null/td-p/314224

I think A is correct because explict-null don't need to config all MPLS router.

The correct is C.

C is the correct answer based on the fact that When an LSR receives an MPLS header in which the label is set to 3, it always POPs the header i.e., it removes the top label. ... An explicit null (Label Value 0 for IPv4), on the other hand, leaves the MPLS header in place until it reaches the egress, preserving the LSP CoS behavior across the entire LSP.

This seems to be wrong. PHP will remove label to egress router, and also remove the ability for the egress router to forward with the CoS in the MPLS network. "When Explicit Null is configured a label value of 0 is signalled to the penultimate router which instructs the router to push an MPLS Shim Label onto packets that are destined for the final hop. In certain cases the requirement for Explicit Null may be desirable, for example to maintain CoS values attributed to the LSP, as the CoS value would normally be discarded at the penultimate router if PHP were enabled."