The answer is that R1’s LDP neighbor is not forming because it lacks an explicitly configured LDP router-id, causing it to default to the IP address of the interface facing R2, which is not reachable from R2’s loopback-based router-id. In MPLS LDP, the router-id must be a stable, reachable IP address—typically a loopback—so that both peers can establish a TCP session on port 646. Without a configured router-id on R1, LDP picks the highest interface IP, which in this case is the directly connected link address; R2, with its forced loopback router-id, cannot reach that interface IP from its own loopback, breaking the neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that LDP session formation depends on IP reachability between router-ids, not just physical adjacency—a common trap is assuming directly connected interfaces are sufficient. Remember the memory tip: “LDP needs a loopback to talk back; without one, the neighbor is gone.”
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. 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.
An MPLS network with EIGRP as the IGP is experiencing label distribution failures. Router R1 shows: 'show mpls ldp neighbor' does not list R2. R1's configuration: mpls ip on interfaces, but no router-id configured. R2's configuration: mpls ldp router-id Loopback0 force. R1 and R2 are directly connected. 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
✓
R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback.
LDP uses a router-id to establish sessions. If R1 does not have a router-id configured, it may use an interface IP that is not reachable from R2, causing the LDP session to fail.
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.
✓
R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback.
Why this is correct
LDP requires a reachable router-id. Without explicit configuration, R1 uses the interface IP, which may not be in R2's routing table.
R2's 'force' keyword causes it to use the loopback even if it is not reachable.
Why it's wrong here
The 'force' keyword ensures the router-id is used, but it does not cause failure.
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.
Trap categories for this question
Keyword trap
The 'force' keyword ensures the router-id is used, but it does not cause failure.
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.
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback. — LDP uses a router-id to establish sessions. If R1 does not have a router-id configured, it may use an interface IP that is not reachable from R2, causing the LDP session to fail.
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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