Areas
Last updated
Last updated
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.
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.
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.
