The answer is that the ACL blocks UDP port 646, which is required for LDP label advertisement messages, preventing label distribution. While LDP establishes its session over TCP port 646, it relies on UDP port 646 for sending link hello messages and, critically, for the actual label advertisement process in many implementations—especially when handling IPv6 prefixes. The ACL permits TCP 646, so the neighbor session appears up, but the UDP-based label exchange is denied, meaning R2 never receives the label bindings from R1, even though R1’s local bindings exist. This scenario tests your understanding of LDP’s dual transport requirements on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming that a successful TCP session guarantees full label distribution. Remember: TCP builds the session, but UDP carries the labels—block UDP 646, and you block the advertisement.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 uses LDP for label distribution with IPv6. Router R1 and R2 are LDP peers. R1 has an IPv6 ACL applied inbound on the interface facing R2 that permits only TCP port 646 (LDP) and denies all other traffic. R2 shows 'show mpls ldp neighbor' indicates the neighbor is up, but 'show mpls forwarding-table' shows no labels for IPv6 prefixes. R1's 'show mpls ldp bindings' shows labels for all prefixes. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL blocks UDP port 646, which is used for LDP label advertisement messages, preventing label distribution.
LDP uses UDP for discovery (hello) and TCP for session. The ACL permits TCP 646, so the session is established. However, LDP also uses UDP for targeted hellos or link hellos, and the ACL blocks UDP 646, preventing proper label exchange. The bindings exist but are not advertised because the UDP-based label advertisement is blocked.
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 ACL blocks UDP port 646, which is used for LDP label advertisement messages, preventing label distribution.
Why this is correct
LDP uses TCP for session but UDP for initial discovery and some label advertisements. Blocking UDP 646 stops label exchange.
LDP session is established but the routers have mismatched label ranges, causing no label bindings.
Why it's wrong here
Label range mismatch would cause session failure, not just missing forwarding entries.
✗
IPv6 CEF is not enabled on R1, preventing label imposition.
Why it's wrong here
CEF is required for MPLS forwarding, but the issue is label distribution.
✗
R2 has an outbound ACL blocking LDP packets, causing asymmetric filtering.
Why it's wrong here
The neighbor is up, so outbound filtering is not the issue.
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.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The ACL blocks UDP port 646, which is used for LDP label advertisement messages, preventing label distribution. — LDP uses UDP for discovery (hello) and TCP for session. The ACL permits TCP 646, so the session is established. However, LDP also uses UDP for targeted hellos or link hellos, and the ACL blocks UDP 646, preventing proper label exchange. The bindings exist but are not advertised because the UDP-based label advertisement is blocked.
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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