Understanding MPLS LDP and PHP (Penultimate Hop Popping):

✔Label Distribution in MPLS LDP:

LDP assignslabels to FECsso that routers can forward packets based on labels rather than IP lookup.

Labels areadvertised downstream(from R4 to R3, R3 to R2, and so on).

✔Penultimate Hop Popping (PHP) Concept:

Theegress LSR (R4)doesnot require an incoming labelfor 4.4.4.0/24.

Instead,R3 (Penultimate LSR) removes the labelbefore forwarding the packet to R4.

R3 forwards the packet as an IP packet, not an MPLS packet, toavoid an unnecessary label lookup at R4.

InMPLS LDP, the label "0" is used to indicate an implicit NULL label (PHP operation).

Why the Answer is "0" (Implicit NULL Label)?

✅Since R4 is the egress LSR, it does not need a label for 4.4.4.0/24.

✅R3 assigns the outgoing label as "0" (Implicit NULL Label)to indicate that the label should be removed before sending to R4.

✅R4 receives the packet as a pure IP packet (no MPLS label).

Why Other Answers Are Incorrect?

❌Any value other than "0"

If R3 assigns a label (e.g., 100), R4 would need to perform anunnecessary MPLS lookup, which is inefficient.

Using "0" ensures PHP occurs, optimizing the forwarding process.

Final Answer: 0

????Reference:Huawei HCIE Datacom – MPLS LDP and Penultimate Hop Popping (PHP)