Comprehensive and Detailed In-Depth Explanation:
1. Understanding MPLS Label Forwarding Information
The output of the display mpls lsp command shows the Label Forwarding Information Base (LFIB) used by the router to forward MPLS packets.
Each entry consists of:
FEC (Forwarding Equivalence Class): The destination IP prefix.
In Label / Out Label: Incoming label (received) and outgoing label (sent).
In IF / Out IF: Incoming and outgoing interfaces.
2. Analyzing the Given LSP Table
FEC (Destination IP)
In Label
Out Label
Outgoing Interface
1.1.1.1/32
3
NULL
-
2.2.2.2/32
NULL
3
GE0/0/1
2.2.2.2/32
1026
3
GE0/0/1
3.3.3.3/32
NULL
1027
GE0/0/1
3.3.3.3/32
1027
1027
GE0/0/1
Key Observations:
NULL means the label is removed (Penultimate Hop Popping - PHP).
Label 3 is the implicit null label, meaning the label is removed before sending to the next router.
For destination 3.3.3.3, the router adds label 1027 before forwarding it to GE0/0/1.
3. Evaluating Each Answer Option
Option A: "Before forwarding a packet destined for 2.2.2.2, the device adds label 3 to the packet." → Incorrect.
The output shows that packets destined for 2.2.2.2 have the NULL (label 3) entry, which means the label is removed before forwarding, not added.
Option B: "Before forwarding a packet destined for 1.1.1.1, the device adds label 3 to the packet." → Incorrect.
The entry 1.1.1.1 → 3/NULL means the label is removed (PHP), so the device does not add label 3.
Option C: "Before forwarding a packet destined for any IP address, the device removes labels from the packet." → Incorrect.
The device removes labels only for some destinations (e.g., 1.1.1.1 and 2.2.2.2), but it adds label 1027 for 3.3.3.3.
Not all IPs have their labels removed.
Option D: "Before forwarding a packet destined for 3.3.3.3, the device adds label 1027 to the packet." → Correct.
The entry 3.3.3.3 → NULL/1027 means that the router adds label 1027 before forwarding packets to GE0/0/1.
Final Answer:
Answer: D (Before forwarding a packet destined for 3.3.3.3, the device adds label 1027 to the packet).
HCIP-Datacom-Advanced Routing & Switching Technology References:
Understanding MPLS LDP (Label Distribution Protocol) and LFIB
Implicit Null (Label 3) and Penultimate Hop Popping (PHP)
MPLS Label Swapping and Forwarding Process
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