mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting MPLS L3VPN where traffic from…
A network engineer is troubleshooting MPLS L3VPN where traffic from CE1 to CE2 is being dropped. On PE1, show mpls forwarding-table shows that the label for the BGP next-hop is 'Pop tag' (implicit null). The engineer checks the remote PE and sees that it is advertising implicit null for the BGP next-hop. What is the most likely reason for the traffic drop?
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 remote PE does not have the CE route in its VRF.
Implicit null (pop tag) means the penultimate hop pops the label and forwards the IP packet. If the traffic is dropped, the issue is likely that the remote PE does not have a route to the CE in its VRF, so it drops the IP packet after label pop.
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 remote PE does not have the CE route in its VRF.
Why this is correct
Correct because after label pop, the packet is forwarded as IP; if the remote PE lacks the VRF route, it drops the packet.
- ✗
The penultimate hop is not popping the label correctly.
Why it's wrong here
Incorrect because the label is being popped as per implicit null; the issue is after the pop.
- ✗
The MTU on the link between P and PE2 is too small.
Why it's wrong here
Incorrect because MTU issues would cause fragmentation or drops regardless of label pop; the symptom is specific to missing VRF route.
- ✗
The remote PE is configured with explicit null for the BGP next-hop.
Why it's wrong here
Incorrect because the stem states implicit null is advertised; explicit null would be a different label value.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
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.
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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