An OSPF area is a logical group of routers that share the same link-state database (LSDB). Dividing a large OSPF network into smaller areas reduces LSDB memory use, shortens SPF calculations, and limits LSA flooding and instability to the local area. section 18.1.2 section 18.5
OSPF uses a two-level hierarchy. Area 0 is the backbone, and every non-backbone area must connect to it; traffic between areas passes through area 0. An internal router keeps all OSPF-enabled interfaces in one area, while an area border router (ABR) connects one or more areas to the backbone. section 18.1.2
Area membership has direct operational consequences:
- Use area 0 for single-area OSPF when possible; this simplifies later expansion into a multi-area design. section 18.1.2
- Match the area number on neighboring interfaces; an area mismatch prevents routers from becoming OSPF neighbors. section 18.3.3
- Distinguish intra-area routes from inter-area routes, and distinguish both from external routes outside the OSPF autonomous system. section 18.5
Fixing an area mismatch
R1 uses `ip ospf 1 area 0`, while R2 initially uses `ip ospf 1 area 1`; R2 reports a mismatched area ID and has no neighbor output. Changing R2 to `ip ospf 1 area 0` restores a FULL adjacency.
