- A
The LDP neighbor IP address configured on R1 does not match R2's LDP transport address, causing MD5 authentication to fail.
The 'mpls ldp neighbor' command expects the neighbor's LDP transport address (usually the router ID). If R1 uses 10.0.0.2 but R2's transport address is different (e.g., 10.0.0.3), the TCP connection uses a different IP, and MD5 authentication fails because the password is associated with the wrong IP.
- B
The password is not configured globally under 'mpls ldp password' on both routers.
Why wrong: The 'mpls ldp neighbor' command with password is sufficient; global password is not required.
- C
The MPLS LDP session is using a different port number, causing authentication to be ignored.
Why wrong: LDP uses TCP port 646; there is no alternative port.
- D
The interface between R1 and R2 has 'mpls ip' disabled.
Why wrong: If mpls ip were disabled, LDP would not form at all, but the authentication failure indicates the session is attempted.
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. 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 is experiencing label distribution failures. Router R1 (LSR) has the following configuration: mpls ldp neighbor 10.0.0.2 password cisco. Router R2 shows: 'show mpls ldp neighbor' lists R1 as 'Oper Down' with reason 'TCP MD5 authentication failure'. R1's 'show mpls ldp neighbor' shows R2 as 'Oper Down' with the same reason. Both routers have the same password configured. 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 neighbor IP address configured on R1 does not match R2's LDP transport address, causing MD5 authentication to fail.
LDP uses TCP for session establishment, and MD5 authentication is configured via the 'mpls ldp neighbor' command. However, the password must match on both ends, and the command must specify the correct neighbor IP. If the IP address specified is incorrect (e.g., using a loopback IP instead of the transport address), the authentication will fail. Additionally, the 'mpls ldp password' command under the interface or global configuration may be required. In this scenario, the root cause is likely that the neighbor IP in the command does not match the actual LDP transport address (e.g., R1 uses 10.0.0.2 but R2's LDP transport address is 10.0.0.3).
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.
- ✓
The LDP neighbor IP address configured on R1 does not match R2's LDP transport address, causing MD5 authentication to fail.
Why this is correct
The 'mpls ldp neighbor' command expects the neighbor's LDP transport address (usually the router ID). If R1 uses 10.0.0.2 but R2's transport address is different (e.g., 10.0.0.3), the TCP connection uses a different IP, and MD5 authentication fails because the password is associated with the wrong IP.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The password is not configured globally under 'mpls ldp password' on both routers.
Why it's wrong here
The 'mpls ldp neighbor' command with password is sufficient; global password is not required.
- ✗
The MPLS LDP session is using a different port number, causing authentication to be ignored.
Why it's wrong here
LDP uses TCP port 646; there is no alternative port.
- ✗
The interface between R1 and R2 has 'mpls ip' disabled.
Why it's wrong here
If mpls ip were disabled, LDP would not form at all, but the authentication failure indicates the session is attempted.
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
Command / output trap
The 'mpls ldp neighbor' command with password is sufficient; global password is not required.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The LDP neighbor IP address configured on R1 does not match R2's LDP transport address, causing MD5 authentication to fail. — LDP uses TCP for session establishment, and MD5 authentication is configured via the 'mpls ldp neighbor' command. However, the password must match on both ends, and the command must specify the correct neighbor IP. If the IP address specified is incorrect (e.g., using a loopback IP instead of the transport address), the authentication will fail. Additionally, the 'mpls ldp password' command under the interface or global configuration may be required. In this scenario, the root cause is likely that the neighbor IP in the command does not match the actual LDP transport address (e.g., R1 uses 10.0.0.2 but R2's LDP transport address is 10.0.0.3).
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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Last reviewed: Jun 18, 2026
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