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300-410 Practice Question: A DMVPN Phase 3 network with MPLS LDP configured…
A DMVPN Phase 3 network with MPLS LDP configured on the tunnel interfaces experiences label distribution failures. R1 (hub) and R2 (spoke) have LDP neighborships established, but R2 is not receiving labels for prefixes behind R3 (another spoke). What is the root cause?
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
✓
LDP is not enabled on the tunnel interface of R3, so R2 cannot receive labels for prefixes behind R3.
In DMVPN Phase 3, the tunnel interface is a multipoint interface. LDP uses the primary IP address of the interface for neighbor discovery. If the tunnel interface is configured with 'ip mtu' or 'ip tcp adjust-mss' that differs between routers, or if the LDP hello packets are not reaching the other routers due to NHRP resolution issues, label distribution may fail. The most common cause is that LDP is not enabled on the tunnel interface or the label space is not correctly configured for DMVPN.
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 is not enabled on the tunnel interface of R3, so R2 cannot receive labels for prefixes behind R3.
Why this is correct
Correct. LDP must be enabled on all tunnel interfaces for label distribution. If R3 does not have 'mpls ip' on its tunnel interface, it will not distribute labels.
- ✗
R2 has a higher LDP router ID, causing it to become the LDP session initiator.
Why it's wrong here
Incorrect. LDP router ID does not affect label distribution; both routers must have LDP enabled.
- ✗
NHRP is not resolving R3's tunnel IP to its physical IP, preventing LDP hello packets from reaching R3.
Why it's wrong here
Incorrect. LDP uses the tunnel interface IP, which is resolved by NHRP. If NHRP is working, LDP should work. The issue is LDP configuration.
- ✗
The MPLS label range on R2 is exhausted, preventing new label bindings.
Why it's wrong here
Incorrect. Label range exhaustion would affect all label distribution, not just for R3.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 3
DMVPN Phase 3 is a Cisco networking technology that allows branch offices to connect directly to each other without always going through a central hub, but with smarter routing that lets the hub control the traffic paths more efficiently.
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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