mediumMultiple Choice
350-401 Practice Question: An engineer is designing an MPLS L3VPN service…
An engineer is designing an MPLS L3VPN service for a customer that requires overlapping IP addresses between two sites. The customer uses OSPF as the PE-CE protocol. The engineer configures VRFs on the PE routers and assigns unique route distinguishers (RDs) and route targets (RTs). However, the customer reports that routes from one site are not being installed in the other site's VRF. What is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates confuse the role of route distinguishers (RDs) with route targets (RTs), thinking that unique RDs are sufficient for route exchange, when in fact RTs control the import/export policy between VRFs.
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 route-target export on PE1 does not match the route-target import on PE2.
In MPLS L3VPN, route targets (RTs) control the import and export of VPN routes between VRFs. For routes from one site to be installed in another site's VRF, the route-target export on the exporting PE must match the route-target import on the importing PE. If they do not match, the routes are not imported, even if route distinguishers (RDs) are unique and OSPF is properly configured.
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 export on PE1 does not match the route-target import on PE2.
Why this is correct
In MPLS L3VPN, route targets (RTs) are BGP extended communities that control the redistribution of VPN routes between VRFs. The exporting PE attaches an export RT to a VPNv4 route; the importing PE only places that route into a VRF if the route's RT matches the VRF's import RT. In this scenario, PE1's export RT does not match PE2's import RT, so even though the VPNv4 route reaches PE2 via BGP, it is not installed in the VRF routing table, leaving the prefix unreachable.
- ✗
The overlapping IP addresses cause a routing loop in OSPF.
Why it's wrong here
Overlapping IP addresses cannot cause an OSPF loop because each VRF runs its own isolated OSPF instance with a separate routing table and link-state database. The PE maintains completely independent forwarding contexts per VRF, and routes learned in one VRF are never redistributed into another by OSPF. Since OSPF's SPF algorithm computes routes within a single VRF's topology, a duplicate prefix in another VRF is simply not visible to that instance, so no loop path can form.
- ✗
OSPF cannot carry overlapping prefixes in different VRFs.
Why it's wrong here
OSPF is designed to run multiple independent processes or instances, and each VRF can have its own OSPF process with its own interface associations, router ID, and LSDB. There is no protocol restriction preventing overlapping prefixes in different VRFs; in fact, that is a primary use case for MPLS L3VPN, where many customers reuse the same RFC 1918 space. The PE simply associates each OSPF process with a VRF, and the separate routing tables keep the overlapping prefixes fully isolated.
- ✗
The route distinguisher is not unique between the two sites.
Why it's wrong here
The route distinguisher (RD) only needs to be unique within a single PE; it does not have to be globally unique across different PEs or sites. The RD is prepended to the IPv4 prefix to create a VPNv4 address, and because it is part of the address itself, reuse of the same RD at another site is harmless—the routes are still distinct entries in the BGP VPNv4 table. The actual decision to import a VPNv4 route into a VRF is made based on RT matching, not RD matching, so an RD mismatch is never the reason for a missing route.
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
VLANs and Spanning Tree Protocol Concepts
Key term
Virtual Routing and Forwarding
Virtual Routing and Forwarding (VRF) is a technology that allows a single physical router to operate like multiple independent routers by keeping separate routing tables and forwarding decisions for each instance.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.