- A
The LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings.
If the router-id is not reachable, LDP session may form using the link address, but label bindings for prefixes learned via OSPF may fail because the transport address is not routable.
- B
The mpls ldp label allocation is configured as 'per-prefix' instead of 'per-interface'.
Why wrong: Label allocation mode does not prevent bindings from being exchanged; it affects how labels are assigned.
- C
The OSPF process on R1 has a route-map filtering the loopback route.
Why wrong: If the loopback is filtered, the router-id would not be reachable, causing LDP session failure, but the session is OPERATIONAL.
- D
The LDP session is using the interface IP address as transport, but OSPF is not advertising the interface network.
Why wrong: The interface network is directly connected, so it is always reachable; the issue is with the router-id.
Quick Answer
The answer is that R1's LDP router-id is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to fail in exchanging label bindings. This is correct because while the LDP session itself is established—evidenced by the OPERATIONAL neighbor state—the actual label binding exchange requires the LDP router-id (1.1.1.1) to be reachable as the transport address. Without OSPF advertising R1's loopback0, R2 cannot route to 1.1.1.1, so LDP cannot complete the label mapping process for prefixes learned via OSPF, even though the TCP session on port 646 is up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that LDP neighbor adjacency and label binding exchange are separate phases; a common trap is assuming an OPERATIONAL neighbor state guarantees label distribution. Remember the memory tip: "Session up, labels stuck? Check the router-id's route."
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is experiencing label distribution failures. Router R1 is an LSR connected to R2. R1's show mpls ldp neighbor shows R2 in OPERATIONAL state, but show mpls ldp bindings shows no label bindings for prefixes learned via OSPF from R2. R1's mpls ldp router-id is 1.1.1.1, and R2's is 2.2.2.2. The OSPF process on R1 advertises the loopback0 interface with ip address 1.1.1.1 255.255.255.255, and R2's loopback0 is 2.2.2.2. The link between them is 192.168.1.0/30. What is the root cause?
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 LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings.
R1 shows R2 as an LDP neighbor in OPERATIONAL state, meaning the LDP session (TCP port 646) is established. However, no label bindings are exchanged for OSPF-learned prefixes from R2. LDP uses the router-id (1.1.1.1) as the transport address for label binding exchange. If OSPF does not advertise R1's loopback0 (1.1.1.1/32), R2 cannot reach this address, so LDP cannot complete the label mapping exchange, even though the neighbor session is up. The correct root cause is that R1's LDP router-id is not reachable from R2.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings.
Why this is correct
If the router-id is not reachable, LDP session may form using the link address, but label bindings for prefixes learned via OSPF may fail because the transport address is not routable.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The mpls ldp label allocation is configured as 'per-prefix' instead of 'per-interface'.
Why it's wrong here
Label allocation mode does not prevent bindings from being exchanged; it affects how labels are assigned.
- ✗
The OSPF process on R1 has a route-map filtering the loopback route.
Why it's wrong here
If the loopback is filtered, the router-id would not be reachable, causing LDP session failure, but the session is OPERATIONAL.
- ✗
The LDP session is using the interface IP address as transport, but OSPF is not advertising the interface network.
Why it's wrong here
The interface network is directly connected, so it is always reachable; the issue is with the router-id.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between LDP neighbor adjacency (which can form using link-local addresses) and the actual exchange of label bindings, which requires the LDP router-id to be reachable via the IGP; candidates mistakenly assume an OPERATIONAL neighbor state guarantees full label exchange.
Detailed technical explanation
How to think about this question
LDP uses the 'transport address' (typically the LDP router-id) to establish TCP connections for label binding exchange. When the LDP router-id is not reachable via the IGP (e.g., OSPF), the LDP session may still form if the peers use the link-local interface IPs for discovery, but label bindings cannot be exchanged because the transport address is unreachable. This is a common misconfiguration where the loopback interface is not included in the OSPF network statement, causing the LDP router-id to be unreachable from the peer.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings. — R1 shows R2 as an LDP neighbor in OPERATIONAL state, meaning the LDP session (TCP port 646) is established. However, no label bindings are exchanged for OSPF-learned prefixes from R2. LDP uses the router-id (1.1.1.1) as the transport address for label binding exchange. If OSPF does not advertise R1's loopback0 (1.1.1.1/32), R2 cannot reach this address, so LDP cannot complete the label mapping exchange, even though the neighbor session is up. The correct root cause is that R1's LDP router-id is not reachable from R2.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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