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300-410 Practice Question: MPLS LDP neighbors are not forming between two…
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 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
✓
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.
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.
- ✗
The mpls ip command is missing on R2's interface.
Why it's wrong here
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.
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Related to this question
Learn chapter
OSPF Neighbor Adjacency and Basics
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
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