Areas

OSPF uses areas to divide up the network.

Small networks can be single-area without any negative effects on performance.

In larger networks, a single-area design can have negative effects:

• the SPF algorithm takes more time to calculate routes.

• the SPF algorithm requires exponentially more processing power on the routers.

• the larger LSDB takes up more memory on the routers.

• any small change in the network causes every router to flood LSAs and run the SPF algorithm again.

By dividing a large OSPF network into several smaller areas, you can avoid the above negative effects.

Definitions

• An area is a set of routers and links that share the same LSDB.

• The backbone area (area 0) is an area that all other areas must connect to.

• Routers with all interfaces in the same area are called internal routers.

• Routers with interfaces in multiple areas are called area border routers (ABRs).

ABRs maintain a separate LSDB for each area they are connected to. It is recommended that you connect an ABR to a maximum of 2 areas. Connecting an ABR to 3+ areas can overburden the router.

• Routers connected to the backbone area (area 0) are called backbone routers.

• An intra-area route is a route to a destination inside the same OSPF area.

• An inter-area route is a route to a destination in a different OSPF area.

Rules

• OSPF areas should be contiguous.

• All OSPF areas must have at least one ABR connected to the backbone area.

• OSPF interfaces in the same subnet must be in the same area.