Which of the following descriptions of Ingress node forwarding in the MPLS forwarding process are correct? (Multiple choice)

Question

Options

• AAfter receiving the data packet, the ingress node will first check the ILM table to find the Tunnel ID.
• BFind the corresponding NHLFE entry according to the Tunnel ID of the ILM table, and associate
• CCheck the NHLFE table entry to get the outbound interface, next hop, outbound label and label
• DPush the obtained label into the IP packet, process the EXP according to the QoS policy, and

• A
  15% (4)
• B
  8% (2)
• C
  77% (20)

Explanation

At an ingress node, after classifying an incoming IP packet into a Forwarding Equivalence Class (FEC), the node consults the FTN (FEC-to-NHLFE) table - not the ILM - to find the correct NHLFE entry (C), which supplies the outbound interface, next hop, and label operation. The node then pushes the MPLS label onto the IP packet and sets the EXP (Traffic Class) bits according to the configured QoS policy (D), making C and D the correct steps.

Why A is wrong: The ILM (Incoming Label Map) is used at transit and egress nodes to process packets that already carry an MPLS label - not at the ingress node processing a raw IP packet. Ingress uses the FTN table instead.

Why B is wrong: It incorrectly ties the Tunnel ID lookup to the ILM table; at ingress the FTN maps the FEC (not an incoming label) to the NHLFE. The ILM has no role at ingress.

Memory tip: Split the MPLS pipeline by node role - Ingress = FTN → NHLFE → Push, Transit = ILM → NHLFE → Swap, Egress = ILM → Pop. If a question mentions ILM at an ingress step, it's a trap.

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