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300-410 Practice Question: Is troubleshooting IPv6 MPLS LDP neighbor…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.