Overview of OSPF

OSPF is a link-state routing protocol applied to routers grouped into OSPF areas identified by the routing configuration on each routing switch. The protocol uses LSAs transmitted by each router to update neighboring routers regarding its interfaces and the routes available through those interfaces. Each routing switch in an area also maintains a link-state database (LSDB) that describes the area topology. (All routers in a given OSPF area have identical LSDBs.) The routing switches used to connect areas to each other flood summary link LSAs and external link LSAs to neighboring OSPF areas to update them regarding available routes. Through this means, each OSPF router determines the shortest path between itself and a desired destination router in the same OSPF domain (AS.)Routed traffic in an OSPF AS is classified as one of the following:

Intra-area traffic
Inter-area traffic
External traffic

The switches support the following types of LSAs, which are described in RFCs 2328 and 3101:

Table 30: OSPF LSA types
LSA type	LSA name	Use
1	Router link	Describes the state of each interface on a router for a given area. Not propagated to backbone area.
2	Network link	Describes the OSPF routers in a given network. Not propagated to backbone area.
3	Summary link	Describes the route to networks in another OSPF area of the same AS. Propagated through backbone area to other areas.
4	Autonomous System (AS) summary link	Describes the route to an ASBR in an OSPF normal or backbone area of the same AS. Propagated through backbone area to other areas.
5	AS external link	Describes the route to a destination in another AS (external route.) Originated by ASBR in normal or backbone areas of an AS and propagates through backbone area to other normal areas. For injection into an NSSA, ABR converts type-5 LSAs to a type-7 LSA advertising the default route (0.0.0.0/0.)
7	AS external link in an NSSA	Describes the route to a destination in another AS (external route.) Originated by ASBR in NSSA. ABR converts type-7 LSAs to type-5 LSAs for injection into the backbone area.