This 300-410 practice question tests your understanding of mpls operations. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
In an MPLS LDP network, routers R1 and R2 are directly connected via GigabitEthernet0/0. R1 shows 'show mpls ldp neighbor' output: 'Peer LDP Ident: 10.0.0.2:0, Local LDP Ident: 10.0.0.1:0, TCP connection: 10.0.0.2.646 - 10.0.0.1.646, State: OPERATIONAL' but 'show mpls forwarding-table' for prefix 192.168.1.0/24 shows 'No label'. R2 has the same prefix in its routing table. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R2 does not have MPLS enabled on the interface facing the prefix's origin; LDP only labels routes learned via MPLS-enabled interfaces.
LDP neighbors are operational, but label binding may fail if the IGP (e.g., OSPF) has a route to the prefix but LDP does not advertise a label for it. This can occur if the prefix is a connected route on R2 but R2's LDP is not configured to advertise labels for connected routes (e.g., 'no mpls ldp advertise-labels' or route filtering). Alternatively, R2 may have the prefix via a different interface not running MPLS.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
R2 does not have MPLS enabled on the interface facing the prefix's origin; LDP only labels routes learned via MPLS-enabled interfaces.
Why this is correct
If the prefix is learned via an interface without 'mpls ip', LDP will not assign a label, causing R1 to have no label in forwarding table.
R1's MPLS forwarding table is full; no room for new labels.
Why it's wrong here
No indication of resource exhaustion.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
MPLS Operations — This question tests MPLS Operations — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: R2 does not have MPLS enabled on the interface facing the prefix's origin; LDP only labels routes learned via MPLS-enabled interfaces. — LDP neighbors are operational, but label binding may fail if the IGP (e.g., OSPF) has a route to the prefix but LDP does not advertise a label for it. This can occur if the prefix is a connected route on R2 but R2's LDP is not configured to advertise labels for connected routes (e.g., 'no mpls ldp advertise-labels' or route filtering). Alternatively, R2 may have the prefix via a different interface not running MPLS.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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