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300-410 Practice Question: An MPLS network with routers R1, R2, and R3 is…
An MPLS network with routers R1, R2, and R3 is experiencing label distribution failures. R1 and R2 are LDP neighbors, but R2 shows: 'show mpls ldp neighbor' shows R1 in state OPERATIONAL, but 'show mpls forwarding-table' shows no labels for prefixes from R3. R3 is connected to R2 via a different interface. R2 configuration: mpls ip on both interfaces. R1 shows: 'show mpls ldp bindings' includes prefixes from R3. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between LDP neighbor state (OPERATIONAL) and the presence of label bindings in the forwarding table, tricking candidates into thinking that a working LDP session on one link guarantees label exchange on all links.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels.
The root cause is that the interface between R2 and R3 is missing the 'mpls ip' command. Without this command on the link, LDP cannot form a session or exchange label bindings across that interface. Since R2 and R3 are not LDP neighbors, R2 never receives labels for prefixes from R3, even though R1 has learned those bindings via its own LDP session with R2.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels.
Why this is correct
LDP must be enabled on each interface where label exchange is desired; without it, no session forms, and no labels are exchanged.
- ✗
The LDP router ID on R2 is set to a loopback that is not reachable from R3.
Why it's wrong here
If the router ID were unreachable, the LDP session would not establish, but the scenario shows R2 and R3 are connected; the issue is likely the interface configuration.
- ✗
R3 is using a different label distribution protocol, such as TDP.
Why it's wrong here
TDP is deprecated; modern networks use LDP, and the scenario assumes LDP.
- ✗
The MPLS label range on R2 is exhausted, preventing new label bindings.
Why it's wrong here
Label exhaustion would affect all prefixes, not just those from R3.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written by Johnson Ajibi, MSc IT Security
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