OSPF LSA Type for Matching Domain ID Route Import
Short answer
B When PE2 imports BGP routes with a matching domain ID into OSPF, they are advertised to CE2 as Type 3 LSAs.
Question
Key Takeaways
The key trap is confusing Type 3 (inter-area) with Type 5 (external); matching domain IDs force the router to treat imported routes as inter-area summaries.
Learn why Type 3 LSAs are used when importing BGP routes into OSPF with matching domain IDs in H12-891 exams.
Common mistake: Candidates often choose Type 5 LSA because BGP routes are inherently 'external' to OSPF. However, the domain ID match overrides this, reclassifying them as inter-area summaries (Type 3).
Expert Analysis
The scenario describes a PE router (PE2) redistributing BGP routes into OSPF. The critical constraint is that the same domain ID is configured on both PEs. In Huawei's implementation of BGP-LS or specific MPLS VPN route leaking mechanisms involving OSPF, the domain ID determines how external routes are advertised to CE routers.
When the local domain ID matches the remote domain ID in the BGP route context, it indicates that the routes are considered part of the same logical area or domain. Consequently, these routes are treated as inter-area routes rather than fully external AS-external routes. Inter-area routes in OSPF are advertised using Type 3 LSAs (Summary LSAs). If the domain IDs were different or absent, the routes would typically be imported as Type 5 LSAs (AS External LSAs). Type 1 LSAs are for intra-area links, and Type 7 LSAs are specific to NSSA areas, which are not indicated here.
Community comments support this by noting that matching domain IDs preserve the route type as Type 3 for inter-area scenarios. Therefore, the LSA type transmitted to CE2 will be Type 3.
Exam Strategy
Always check for domain ID configurations in MPLS/OSPF integration questions. If domain IDs match, assume inter-area behavior (Type 3); if they differ, assume external behavior (Type 5). This distinction is a common exam discriminator.
Frequently Asked Questions
What happens if domain IDs do not match?
Why not Type 1 LSAs?
Community Votes
100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 2 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 2
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7c3a129 👍 1 Answered BBefore advertising the remote routes sent by BGP to CEs, PEs need to determine the type of OSPF routes (Type 3, Type 5, or Type 7) to be advertised to CEs based on domain IDs. If local domain IDs are the same as or compatible with remote domain IDs in BGP routes, PEs advertise Type 3 routes. Otherwise, Type 5 or Type 7 routes are advertised.
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megamind23 👍 1 Answered BSame domain ID preserves route type: intra/inter-area → Type 3; external → Type 5 (normal area) or Type 7 (NSSA). Different/no domain ID → everything becomes Type 5 only.
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