OSPF: Design

OSPF design requires groups of links to be collected in a contiguous area.

Routers that connect to two or more areas are referred to as Area Border Routers (ABR)

All areas are supposed to connect to Area 0, so ABRs should have at least one interface in Area 0.

It is possible to have routers in between areas that AREN’T area 0, Cisco does not consider these routers as ABRs its simply referred to as an internal router.

Autonomous System Boundary Routers (ASBR) inject external routes into OSPF.

OSPF routers keep an independant and seperate LSDB for each area it is connected too.  When computing best path to a network, SPF is run on each LSDB independantly where the results are combined and injected into the routing table.

Using areas provide the following benefits:

  • Smaller per area LSDBs require less memory
  • Faster SPF computations on smaller LSDBs
  • Link failures in a specific area only require partial LSDB updates.
  • routes can be summarized and filtered at ABRs and ASBRs as needed to reduce the number of active routes in the cores Area 0 routing table.

Transit Networks – are a network over which two or more OSPF routers are neighbors and elected a DR so traffic can transit from one to the other.  The exception to this rule is point to point connections between two routers.

Stub Networks – A subnet on which a router has not formed any neighbor relationships.


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