The answer is that LDP cannot form a session between a VRF interface and a global interface; both peers must share the same VRF or both reside in the global routing table. This is because LDP uses UDP for hello discovery and TCP for session establishment, and the transport address—typically the router ID—is bound to the VRF label space. When R1 sends LDP hellos from its VRF-A interface, those packets are encapsulated within the VRF context, but R2’s LDP process operates in the global table, so the hellos are never recognized or replied to. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of VRF-Lite and MPLS LDP interoperability, often appearing as a trick where both sides appear correctly configured but the VRF mismatch breaks neighbor discovery. A common trap is assuming LDP works across VRF boundaries because IP connectivity exists. Memory tip: “VRF and global LDP never mix—keep your label spaces fixed.”
300-410 VRF-Lite Practice Question
This 300-410 practice question tests your understanding of vrf-lite. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Router R1 is configured for VRF-Lite with MPLS. The interface Gig0/0 is in VRF-A and is running LDP. The LDP neighbor with R2 is not establishing. R1 configuration: mpls ip, mpls label protocol ldp, interface Gig0/0, ip vrf forwarding VRF-A, ip address 10.0.0.1 255.255.255.252. R2 has similar configuration without VRF. The LDP hello packets are sent but not received. 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
✓
LDP cannot form a session between a VRF interface and a global interface; both must be in the same VRF or both in the global table.
LDP uses UDP and TCP, and the transport address is typically the router ID. In a VRF, the LDP session must be established within the same VRF. If R1 has VRF-A and R2 does not, the LDP hellos are sent in the VRF context, but R2's LDP process is in the global table. The hellos are not recognized because the VRF label space is different. LDP requires matching VRF or global configuration.
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.
✓
LDP cannot form a session between a VRF interface and a global interface; both must be in the same VRF or both in the global table.
The MPLS label protocol must be TDP on both sides.
Why it's wrong here
Incorrect: LDP is standard; TDP is deprecated.
✗
The interface IP addresses must be in the same subnet.
Why it's wrong here
Incorrect: They are in the same subnet; the issue is VRF mismatch.
✗
The mpls ip command must be applied under the VRF address-family.
Why it's wrong here
Incorrect: The command is applied on the interface; VRF address-family is for routing protocols.
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
Incorrect: The command is applied on the interface; VRF address-family is for routing protocols.
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.
VRF-Lite — This question tests VRF-Lite — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: LDP cannot form a session between a VRF interface and a global interface; both must be in the same VRF or both in the global table. — LDP uses UDP and TCP, and the transport address is typically the router ID. In a VRF, the LDP session must be established within the same VRF. If R1 has VRF-A and R2 does not, the LDP hellos are sent in the VRF context, but R2's LDP process is in the global table. The hellos are not recognized because the VRF label space is different. LDP requires matching VRF or global configuration.
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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