Refer to the exhibit. Which statement about this output is true?

Question

Exhibit

Options

• AThe router IP 192.168.1.2 sent an implicit null, and the output is from the penultimate LSR.
• BThe adjacent router is the egress LSR and has mpls ldp explicit-null configured.
• CThe adjacent LSR router configured mpls label range 0.
• DThe zero in the second column is the normal behavior of an egress router LSR.

• A
  3% (1)
• B
  64% (23)
• C
  11% (4)
• D
  22% (8)

Explanation

Option B is correct because when an egress LSR has mpls ldp explicit-null configured, it advertises label 0 (IPv4 Explicit Null) to its upstream neighbor instead of the default label 3 (Implicit Null). The "0" visible in the output's label column is that advertised explicit-null label, meaning the adjacent router is the egress LSR and has this feature explicitly enabled to preserve QoS/EXP bit markings all the way to the egress.

Why A is wrong: Implicit null uses label 3, not 0. When implicit null is in play, the penultimate LSR pops the label before forwarding - you'd see label 3 advertised, not 0 in the forwarding table output.

Why C is wrong: A label value of 0 in the output reflects what was advertised by the neighbor, not a local label range configuration. The mpls label range command controls locally assigned label pools and has no bearing on receiving a 0-value label from a peer.

Why D is wrong: The normal/default egress behavior is to advertise label 3 (implicit null), triggering Penultimate Hop Popping (PHP). Label 0 requires deliberate configuration; it is not default behavior.

Memory tip: Think "0 = Explicit, 3 = Implicit." Explicit Null (0) = the egress explicitly wants the labeled packet delivered to it. Implicit Null (3) = the egress implicitly says "pop it before you get here."

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