The OSPF backbone area is area 0, the central area in a two-level hierarchy. Every non-backbone area must connect to area 0, so traffic between non-backbone areas passes through the backbone. In a single-area design, using area 0 simplifies later expansion into multi-area OSPF. section 18.1.2
Key relationships:
- A backbone router has at least one OSPF-enabled interface in area 0. section 18.1.2
- An area border router (ABR) connects one or more non-backbone areas to area 0 and maintains a separate LSDB for each connected area. section 18.1.2
- Multi-area OSPF reduces LSDB size and SPF calculation work while limiting network-change effects to an area. section 18.5
Four-area topology
A multi-area OSPF network can contain area 0 plus areas 1, 2, and 3. Areas 1, 2, and 3 connect to area 0; traffic traveling between any two non-backbone areas uses the backbone.
