- A
The 'mpls ldp discovery transport-address interface' command on R1 causes the transport address to be the link-local address, which is not routable.
Link-local addresses are not routable, so R2 cannot establish a TCP session to R1's transport address.
- B
The MPLS LDP label distribution is disabled on the interface, preventing session establishment.
Why wrong: The 'mpls ip' command enables label distribution; the issue is with the transport address.
- C
The LDP hello interval is mismatched between R1 and R2, causing the session to fail.
Why wrong: Hello interval mismatches do not prevent session establishment; they only affect hello reception.
- D
Router R2 has a firewall blocking TCP port 646, preventing the LDP session.
Why wrong: No firewall is mentioned; the debug shows hello packets are exchanged.
300-410 IPv6 First Hop Security Practice Question
This 300-410 practice question tests your understanding of ipv6 first hop security. 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.
A network engineer is troubleshooting IPv6 MPLS LDP neighbor discovery on a link between Router R1 and Router R2. The LDP session is not forming. Router R1 has the following relevant configuration:
interface GigabitEthernet0/0
ipv6 address 2001:DB8:1::1/64 mpls ip mpls ldp discovery transport-address interface !
Router R2 shows: debug mpls ldp discovery output indicates that R2 is receiving Hello packets from R1, but the LDP session remains in INIT state. 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 'mpls ldp discovery transport-address interface' command on R1 causes the transport address to be the link-local address, which is not routable.
The command 'mpls ldp discovery transport-address interface' on Router R1 forces the LDP transport address to be the interface's IPv6 link-local address. Since link-local addresses are not routable, Router R2 cannot reach this transport address to establish the TCP connection (port 646) required for the LDP session. The session stalls in INIT state because the TCP handshake fails, even though Hello packets (UDP) are exchanged successfully.
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 'mpls ldp discovery transport-address interface' command on R1 causes the transport address to be the link-local address, which is not routable.
Why this is correct
Link-local addresses are not routable, so R2 cannot establish a TCP session to R1's transport address.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The MPLS LDP label distribution is disabled on the interface, preventing session establishment.
Why it's wrong here
The 'mpls ip' command enables label distribution; the issue is with the transport address.
- ✗
The LDP hello interval is mismatched between R1 and R2, causing the session to fail.
Why it's wrong here
Hello interval mismatches do not prevent session establishment; they only affect hello reception.
- ✗
Router R2 has a firewall blocking TCP port 646, preventing the LDP session.
Why it's wrong here
No firewall is mentioned; the debug shows hello packets are exchanged.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the subtle difference between LDP Hello (UDP) and LDP session (TCP) establishment, tricking candidates into thinking that receiving Hellos means the session should form, when in fact the transport address must be routable for the TCP handshake to succeed.
Trap categories for this question
Command / output trap
The 'mpls ip' command enables label distribution; the issue is with the transport address.
Detailed technical explanation
How to think about this question
In IPv6 LDP, the transport address is used by the remote peer to initiate a TCP connection to port 646. By default, LDP uses a routable IPv6 address (e.g., the global unicast or unique local address) as the transport address. The 'transport-address interface' option overrides this to use the interface's link-local address, which is only valid on the local link. This is a common misconfiguration when engineers intend to force a specific address but forget that link-local addresses are not globally reachable. The LDP session will remain in INIT state indefinitely because the TCP SYN from R2 to R1's link-local address will never complete.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'mpls ldp discovery transport-address interface' command on R1 causes the transport address to be the link-local address, which is not routable. — The command 'mpls ldp discovery transport-address interface' on Router R1 forces the LDP transport address to be the interface's IPv6 link-local address. Since link-local addresses are not routable, Router R2 cannot reach this transport address to establish the TCP connection (port 646) required for the LDP session. The session stalls in INIT state because the TCP handshake fails, even though Hello packets (UDP) are exchanged successfully.
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: Jul 4, 2026
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