The answer is that an ACL or Control Plane Policing (CoPP) is blocking LDP UDP port 646 or the subsequent TCP session. This is correct because the `show mpls ldp discovery` output confirms that hello packets are being transmitted and received (`xmit/recv`) on the interface, yet no LDP neighbor adjacency forms. LDP uses UDP port 646 for hello discovery and TCP port 646 for session establishment; if either is filtered, the routers will see each other’s hellos but cannot complete the TCP handshake required to form a neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to differentiate between a Layer 2 connectivity issue and a control-plane filtering problem—a common trap is to assume a missing `mpls ip` command when the real culprit is an implicit deny in an ACL or CoPP policy rate-limiting LDP packets. Memory tip: “Hellos heard, no neighbor—check your ACLs for 646.”
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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.
MPLS LDP neighbors are not forming between two directly connected routers. Router R1 has the following relevant configuration: interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.252 mpls ip Router R2 shows: show mpls ldp neighbor No LDP neighbors show mpls ldp discovery Local LDP Identifier: 10.1.1.1:0 Discovery Sources: Interfaces: GigabitEthernet0/0 (ldp): xmit/recv LDP: no Hello adjacencies 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
✓
An ACL or CoPP is blocking LDP UDP port 646 or TCP session.
LDP hello packets are being sent and received (xmit/recv), but no adjacency forms. This could be due to ACL blocking UDP 646, or CoPP rate-limiting LDP packets. The correct fix is to check ACLs and CoPP policies that may drop LDP hello or session packets.
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.
✓
An ACL or CoPP is blocking LDP UDP port 646 or TCP session.
Why this is correct
LDP uses UDP 646 for hellos and TCP 646 for session; if blocked, no adjacency forms.
Missing mpls ip would prevent sending hellos, but R2 shows xmit/recv, so it is enabled.
✗
The LDP router ID is not reachable; check loopback interfaces.
Why it's wrong here
LDP router ID must be reachable; if not, session fails, but hellos still form adjacency.
✗
The label distribution method is different; use 'label distribution cdp' instead.
Why it's wrong here
Label distribution method is not relevant to LDP adjacency.
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
Missing mpls ip would prevent sending hellos, but R2 shows xmit/recv, so it is enabled.
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.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: An ACL or CoPP is blocking LDP UDP port 646 or TCP session. — LDP hello packets are being sent and received (xmit/recv), but no adjacency forms. This could be due to ACL blocking UDP 646, or CoPP rate-limiting LDP packets. The correct fix is to check ACLs and CoPP policies that may drop LDP hello or session packets.
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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