300-410 Infrastructure Services Practice Question
A network engineer is deploying MPLS Layer 3 VPNs on a Cisco IOS XE PE router. The customer VRF CUST_A uses OSPF as the PE-CE routing protocol. The engineer must ensure that OSPF routes from the customer are redistributed into MP-BGP and that the OSPF domain ID is preserved across the MPLS backbone. Which configuration step is required on the PE router?
⚠ Common exam trap
Test-takers frequently confuse route targets or sham links with the OSPF domain ID mechanism, which is a specific OSPF process parameter.
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
✓
Configure route redistribution from OSPF into BGP under the VRF address family and set a domain ID under router ospf with the same value on all PE routers.
Preserving the OSPF domain ID requires redistributing OSPF into MP-BGP and configuring a consistent domain ID under the OSPF process on all PE routers. This ensures that routes carry the domain identifier, which prevents routing loops and maintains OSPF route integrity across the MPLS VPN. Other options either misapply features or do not address domain ID preservation.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the OSPF process with the capability vrf-lite command and redistribute connected routes into BGP.
Why it's wrong here
The capability vrf-lite command is used when OSPF runs on a PE router without MPLS, typically in a VRF-lite scenario. It does not preserve domain IDs across an MPLS L3VPN. Redistributing connected routes also does not address OSPF domain ID propagation, so this option does not meet the requirement.
- ✗
Configure a route target export and import under the VRF and enable OSPF as the provider core routing protocol.
Why it's wrong here
Route targets control VPN route distribution but do not preserve the OSPF domain ID. Enabling OSPF in the provider core is not typical; the core uses MPLS and BGP. This option misidentifies the mechanism for domain ID preservation, which is a specific OSPF process setting, not a route target attribute.
- ✗
Configure a sham link between PE routers and set the OSPF network type to point-to-point on the PE-CE link.
Why it's wrong here
A sham link is used to create a backdoor path for OSPF between sites over the MPLS backbone, but it does not preserve the OSPF domain ID. Changing the network type on the PE-CE link affects adjacency formation but is unrelated to domain ID propagation. This option addresses a different design goal.
- ✓
Configure route redistribution from OSPF into BGP under the VRF address family and set a domain ID under router ospf with the same value on all PE routers.
Why this is correct
To preserve the OSPF domain ID across the MPLS backbone, the PE router must redistribute OSPF into MP-BGP and use the domain ID feature. Configuring the same domain ID under router ospf for the VRF on all PE routers ensures that OSPF routes retain their domain identity, preventing loops and allowing proper route redistribution into BGP.
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
MPLS L3VPN Basics
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
MPLS Layer 2 VPN
An MPLS Layer 2 VPN is a virtual private network technology that connects two or more customer sites at the data link layer, allowing them to communicate as if they were on the same local Ethernet segment, while using a service provider’s MPLS backbone.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.