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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 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using OSPF with the CEs and MP-BGP between them. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route as well. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 but the output interface is 'no route'. 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 PE2 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core.
CEF has a next-hop but no output interface because the recursive routing table lookup for the BGP next-hop (192.168.1.2) fails. The IGP (OSPF or IS-IS) does not have a route to the PE2 loopback, so CEF cannot resolve the adjacency.
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 OSPF process on PE1 is not redistributing connected routes.
Why it's wrong here
Incorrect because the IGP route to the PE2 loopback is missing, not the CE routes.
- ✓
The PE2 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core.
Why this is correct
Correct: The BGP next-hop must be reachable via IGP for CEF to resolve the output interface.
- ✗
The VRF route-target import is misconfigured.
Why it's wrong here
Incorrect because the route is present in the VRF routing table.
- ✗
MPLS is not enabled on the core-facing interfaces.
Why it's wrong here
Incorrect because the issue is IGP reachability, not MPLS label switching.
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
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.
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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Same concept, more angles
1 more way this is tested on 300-410
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Variation 1. An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, but the show ip route vrf CUSTOMER command does not have this route. The show ip bgp vpnv4 all 10.2.2.0/24 command on PE1 shows the route is received but not best. What is the most likely cause?
hard- A.The route-target import on PE1 is missing.
- ✓ B.The BGP next-hop (PE2 loopback) is not reachable in the global routing table.
- C.The VRF on PE1 has a different route-target export.
- D.The MP-BGP session is using an incorrect address family.
Why B: The route is received but not marked as best, so it is not installed in the routing table. Common reasons include the route being suppressed due to a higher AD from another source or the next-hop being unreachable. In this scenario, the most likely cause is that the BGP next-hop is not reachable in the global routing table.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.