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IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

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?

Question 1hardmultiple choice

Read the full MPLS explanation →

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.

Full breakdown with real-world context →

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.

LDP uses transport addresses for session establishment. If the transport address is not reachable or if there is a mismatch in the LDP hello parameters, the session may not form. The correct answer identifies that the transport address on R1 is set to the interface address, but R2 expects a different transport address (e.g., loopback), causing a mismatch that prevents the TCP session from establishing.

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 '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
OSPF neighbours must agree on key parameters.
✗
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: 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 ip' command enables label distribution; the issue is with the transport address.

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?

IPv6 First Hop Security — This question tests IPv6 First Hop Security — OSPF neighbours must agree on key parameters..

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. — LDP uses transport addresses for session establishment. If the transport address is not reachable or if there is a mismatch in the LDP hello parameters, the session may not form. The correct answer identifies that the transport address on R1 is set to the interface address, but R2 expects a different transport address (e.g., loopback), causing a mismatch that prevents the TCP session from establishing.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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