Refer to the exhibit. After implementing a new design for the network, a technician reviews the pictures CLI output as part of the MOP. Which two elements describe what the technician can ascertain from the ImpNull output? (Choose two.)
A.
Ultimate Hop Popping is in use for the prefix displayed.
B.
Penultimate Hop Popping is in use for the prefix displayed.
C.
Label 0 is used for the prefix displayed, but will not be part of the MPLS label stack for packets destined for 192.168.10.10.
D.
Label 3 is in use for the prefix displayed and will be part of the MPLS label stack for packets destined for 192.168.10.10.
E.
Label 0 is used for the prefix displayed and will be part of the MPLS label stack for packets destined for 192.168.10.10.
The label value 3 signals the use of the implicit NULL label, the label 3 will never be seen as a label in the label stack of an MPLS packet. That is why it is called the implicit NULL label.
The label value 0 is the explicit NULL label. Egress LSR receives labelled packets with a label of value 0 as the top label.
100% correct answer is: B
Answers A, C, D, E are incorrect. If answer D would be formulated as follows: "Label 3 is in use for the prefix displayed and will NOT be part of the MPLS label stack for packets destined for 192.168.10.10." - then it would be correct.
Assuming, that this is a badly formulated answer by CISCO, where I would have to provide 2 answers - I would choose: B, D
I completell agree that the other possible answer is not in the options, implicit null is signaled via the label 3 from the egress LSR, but it is NOT part of the stack, due to PHP, the Penultimate router send a pure IP packet to the egress LSR, as such No label is used for this final hop.
Came across this question in another dump, and option D correctly reads:
D. Label 3 is in use for the prefix displayed and will NOT be part of the MPLS label stack for packets destined for 192.168.10.10.
A value of 0 represents the "IPv4 Explicit NULL Label".
This label value is only legal at the bottom of the label
stack. It indicates that the label stack must be popped,
and the forwarding of the packet must then be based on the
IPv4 header.
To be sure Answer is B and C.
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)
References:
https://community.cisco.com/t5/mpls/implicit-null-and-explicit-null/td-p/1496792
https://networkzblogger.com/2017/03/21/mpls-explicit-and-implicit-null-label/
Correct answer is B and C
Implicit Null means label 0 is used for the last hop, but it is not actually pushed.
Explicit null is when label 0 is pushed for PHP.
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