7.2 OSPF Multi-Area Architecture and LSA Types

Why divide OSPF into areas?

An OSPF area limits the scope of detailed topology information. In a small lab, one area may be sufficient. As a network grows, multiple areas can reduce the amount of topology state each router must process and permit controlled advertisement of reachable prefixes across area boundaries.

OSPF uses a two-level hierarchy centered on Area 0, also written 0.0.0.0. Non-backbone areas connect to the backbone, physically or through supported logical mechanisms such as virtual links. Normal inter-area routing follows this hierarchy. It is not a license to connect arbitrary areas and assume that any topology will work.

An area boundary changes routing-information scope; it does not encrypt customer packets or create a separate customer VPN.

Router roles

Roles can overlap. An ABR attached to Area 0 is also a backbone router. A router can be both an ABR and an ASBR if it connects areas and redistributes external routes. Do not equate ASBR with “a router running BGP”: it must actually introduce external reachability into OSPF to serve that role.

An ABR calculates routes using each area's information. It advertises suitable reachability into other areas rather than forwarding the originating area's complete router/link topology unchanged.

LSA types and their scope

A Link-State Advertisement describes information stored in the LSDB. The foundational OSPFv2 types are:

Types 1 and 2 describe detailed local-area topology. They are not flooded unchanged into another area. Types 3 and 4 also have area flooding scope: an ABR can originate new summary advertisements for another area. Saying that every Type 3 LSA itself travels unchanged through all areas would be incorrect.

Type 5 advertisements can cross ordinary area boundaries, but special area types restrict them. A stub area excludes Type 5 external advertisements; NSSA operation uses Type 7 internally with specific translation behavior. These extensions explain why “every router in the AS has every LSA” is too broad.

A three-area example

R1 is internal to Area 1. ABR-A connects Area 1 to Area 0. ABR-B connects Area 0 to Area 2, where R4 is internal. R1 advertises its local link information in Area 1. ABR-A derives a reachable prefix and advertises it toward the backbone. ABR-B can advertise appropriate inter-area reachability into Area 2.

R4 can learn a route to R1's subnet without learning the detailed topology of every link in Area 1. Its next hop points toward its local route to ABR-B. R1 and R4 need not have identical LSDBs because they belong to different areas.

If R1's subnet is also advertised by an external routing source, OSPF route-type rules and the router's overall route selection matter. Do not compare only a displayed cost without identifying whether a route is intra-area, inter-area, or external.

Summarization with explicit boundaries

An ABR can summarize suitable component networks at an area boundary. Four /24 networks—172.16.0.0/24 through 172.16.3.0/24—fit exactly into 172.16.0.0/22. The /22 has mask 255.255.252.0 and covers 1,024 addresses.

This summary reduces the advertisements seen elsewhere. It does not create missing internal routes. If one component disappears, other components may keep the summary active, so traffic for the missing portion still reaches the summarizing boundary. A suitable discard route prevents that traffic from looping through a default route.

Standard OSPF range metric behavior uses the maximum cost among included component networks unless applicable configuration changes the result. For costs 10, 10, 20, and 15, that value is 20, not 10. Never substitute an invented “lowest component cost” rule.

External route costs

OSPF external route types include E1 and E2. E1 incorporates internal cost to reach the external forwarding point as well as the advertised external metric. E2 primarily compares the external metric, using internal distance when the relevant conditions tie.

For example, two otherwise comparable E1 exits advertise external cost 20. If the internal costs are 10 and 30, their totals are 30 and 50. The lower total is preferred. This arithmetic is different from assuming that all external routes simply add hop counts.

Inspect what was learned

Use show router ospf database to inspect advertisements and their area, type, advertising router, and identifiers. Use the route table to inspect selected reachability. A database record is not itself proof that its route won selection against every other source.

Troubleshoot inter-area reachability by checking backbone connectivity, interface area membership, ABR advertisements, summaries, and the actual destination prefix. An adjacency can be Full while an area boundary or summary policy still prevents the required route from appearing.

Primary reference

OSPFv2 specification, checked October 20, 2026. Additional area-type and vendor configuration details require the matching Nokia release manual.