Quick Answer
The answer is the OSPF Domain ID extended community. This BGP extended community is used to signal the OSPF domain identifier between PE routers in an MPLS L3VPN, enabling the receiving PE to determine whether a redistributed OSPF route originated from the same OSPF domain or a different one. When the domain IDs match, the route can be advertised as an intra-area or inter-area route; when they differ, a Type 5 LSA is required, preserving OSPF route type semantics across the MPLS backbone. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of how OSPF inter-domain route propagation works in a Layer 3 VPN context, often appearing in questions about route redistribution and LSA types. A common trap is confusing the OSPF Domain ID with the Route Distinguisher or Route Target—remember that the Domain ID specifically controls OSPF route type behavior, not VPN routing. Memory tip: think “Domain ID decides the LSA type”—same domain, intra/inter-area; different domain, Type 5.
This 350-501 practice question tests your understanding of mpls and segment routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which BGP extended community is used to signal the OSPF domain ID between PE routers in an MPLS L3VPN when OSPF is the PE-CE protocol?
Site of Origin
Why wrong: Site of Origin (SoO) is used to prevent routing loops.
OSPF Domain ID
This community specifically carries the OSPF domain ID.
OSPF Route Type
Why wrong: OSPF Route Type carries the OSPF route type (intra/inter/type1/type2), not the domain ID.
Route Target
Why wrong: Route Target is used for VPN route import/export, not OSPF domain ID.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
OSPF Domain ID
In an MPLS L3VPN where OSPF is the PE-CE protocol, the OSPF Domain ID extended community is used to signal the OSPF domain identifier between PE routers. This allows the receiving PE to determine whether the OSPF route originated from the same OSPF domain (and thus should be redistributed as an intra-area or inter-area route) or from a different domain (requiring a Type 5 LSA). The OSPF Domain ID is carried as a BGP extended community (type 0x0005 or 0x8005) and is critical for maintaining OSPF route type semantics across the MPLS backbone.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Site of Origin
Why it's wrong here
Site of Origin (SoO) is used to prevent routing loops.
OSPF Domain ID
Why this is correct
This community specifically carries the OSPF domain ID.
Related concept
Read the scenario before looking for a memorised answer.
OSPF Route Type
Why it's wrong here
OSPF Route Type carries the OSPF route type (intra/inter/type1/type2), not the domain ID.
Route Target
Common exam traps
Cisco often tests the confusion between the OSPF Domain ID and the OSPF Route Type extended communities, where candidates mistakenly think the Route Type carries the domain information, but in reality the Route Type only encodes the OSPF path type and metric, while the Domain ID identifies the originating OSPF domain.
Detailed technical explanation
The OSPF Domain ID extended community is defined in RFC 4577 and uses a 4-byte value (typically the OSPF process Router ID or a manually configured domain ID). When a PE receives an OSPF route from a CE, it attaches this community to the corresponding VPNv4 route; the remote PE compares the received Domain ID with its local OSPF domain ID to decide whether to generate a Type 3 LSA (same domain) or Type 5 LSA (different domain). A subtle behavior is that if the Domain IDs match, the route is advertised as an inter-area route (Type 3) even if the original route was intra-area, which can affect path selection in multi-homed scenarios.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: OSPF Domain ID — In an MPLS L3VPN where OSPF is the PE-CE protocol, the OSPF Domain ID extended community is used to signal the OSPF domain identifier between PE routers. This allows the receiving PE to determine whether the OSPF route originated from the same OSPF domain (and thus should be redistributed as an intra-area or inter-area route) or from a different domain (requiring a Type 5 LSA). The OSPF Domain ID is carried as a BGP extended community (type 0x0005 or 0x8005) and is critical for maintaining OSPF route type semantics across the MPLS backbone.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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