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300-410 Practice Question: Is troubleshooting an MPLS L3VPN where CE1…
A network engineer is troubleshooting an MPLS L3VPN where CE1 (192.168.1.0/24) cannot reach CE2 (192.168.2.0/24). The PE routers are running OSPF with the CEs. On PE1, the VRF configuration includes route-target import and export 100:100. The show ip vrf detail command on PE1 shows the VRF is active, but the CE1 loopback is not present in the VRF routing table. The show ip route vrf CUSTOMER command on PE1 shows only directly connected interfaces. What is the most likely 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 OSPF process on PE1 is not configured under the VRF.
The CE routes are not being learned via OSPF into the VRF because OSPF process is not associated with the VRF. Without the 'router ospf <pid> vrf <name>' command, OSPF runs in the global routing table and does not populate the VRF.
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 route-target import on PE1 is misconfigured.
Why it's wrong here
Incorrect because route-target import/export is for VPNv4 route exchange between PEs, not for CE-PE route learning.
- ✓
The OSPF process on PE1 is not configured under the VRF.
Why this is correct
Correct: OSPF must be configured with 'router ospf <pid> vrf CUSTOMER' to populate the VRF routing table.
- ✗
The CE1 interface is not in the VRF.
Why it's wrong here
Incorrect because the VRF shows active and directly connected routes, meaning the interface is assigned to the VRF.
- ✗
The MP-BGP session between PE1 and PE2 is down.
Why it's wrong here
Incorrect because the issue is local CE route learning, not VPNv4 route exchange.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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
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