hardMultiple ChoiceObjective-mapped
300-410 Practice Question: In an MPLS network, routers R1 and R2 are LDP…
In an MPLS network, routers R1 and R2 are LDP neighbors with BFD enabled. R1#show mpls ldp neighbor shows R2 as 'Oper Down' for the BFD session. R2#show bfd neighbors shows the session with R1 as 'Down'. R1 and R2 have matching 'mpls ldp router-id' loopback0. The link between them is up. 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
✓
The LDP router-id must be the IP address of the interface where BFD is enabled.
BFD for LDP requires that the LDP session be established over the same interface as the BFD session. If the LDP router-id is not the interface IP used for BFD, the BFD session may fail. Here, the loopback0 is used as LDP router-id, but BFD is configured on the physical link, causing mismatch.
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 LDP router-id must be the IP address of the interface where BFD is enabled.
Why this is correct
BFD for LDP requires the LDP session to use the same source IP as the BFD session; otherwise, BFD cannot monitor the LDP session.
- ✗
BFD is not supported with LDP on this platform.
Why it's wrong here
BFD is supported with LDP on most platforms.
- ✗
The BFD interval is too low for LDP.
Why it's wrong here
BFD interval does not cause session down state; it affects detection time.
- ✗
LDP must be configured with 'no bfd' to work properly.
Why it's wrong here
Removing BFD would not solve the root cause.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
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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