An OSPF area is a logical grouping of OSPF enabled interfaces across multiple routers.
The area is identified by a number or dotted decimal that is assigned at an interface level. The interface can only belong to one area.
The area number is included in OSPF Hello packets that are sent by the router.
The field in the Hello Packet is a 32-bit field, that can be formatted with a number (0 to 4294967295) or dotted decimal (0.0.0.0 – 255.255.255.255)
A router with a number on one side and a dotted decimal on the other can still form an adjacency.
Single Area vs Multi Area
All routers within the area maintain an identical link-state database (LSDB)
Using a single area in an OSPF topology can keep things simplified but there are disadvantages:
- The SPF algorithm will need to run when a link flaps in the area
- The LSDB will continue to increase in size, consuming more memory and taking long to re-compute
- There is no summarisation of routes
Splitting up an OSPF domain into multiple areas will keep the size of the link state database manageable.
A router will multiple interfaces in multiple areas will have multiple link state bases, one for each area. The internal topology of one area will not be visible in the other areas database. If a link state change occurs on one area, the other area will be unaware of that internal change and will not need to perform any recalculation of routes.
The first area on a router, area 0, is a special type of area. It is known as the backbone area. In an OSPF design, all areas must connect to Area 0. There is an expectation in OSPFs design that all areas will advertise their routing information into Area 0, and Area 0 will advertise all routing information to other areas.
Area Border Routers (ABR)
An Area Border Router is an OSPF enabled router that as an interface in Area 0 and another interface in another OSPF area.
An Area Border Router is responsible for taking routers from one area and and advertising them into the other area.