OSPF summary


OSPF uses IP protocol 89 to communicate.

OSPF also uses two multicast groups: AllSPFRouters =, and ALLDRouters =

An OSPF internetwork can be divided into up to 2^32 OSPF areas. A network with one area will have only area 0, which is mandatory. Areas are used to limit the scope of LSAs; when an LSA is sent it floods its entire area but, depending on what it is, need not also flood other areas. Areas also help conserve router memory and improve performance by limiting the size of the OSPF link state database.

When there is more than one area, the rule is that area 0 is the hub area: no two other areas can communicate without doing so via area 0.

LSA types

OSPF routers originate link state advertisements (LSAs) to describe the local environment. LSAs are flooded within the area using multicast. Any topology change in the area will trigger a new flood of LSAs. All LSAs, taken together, form the link state database. Every router in an area will have the exact same link state database.

Type 1: Router-LSA

Type 2: Network-LSA

Type 3: Summary-LSA

Type 5: AS-external-LSA

Type 4: ASBR-summary-LSA

Type 7: Type-7-LSA

Type 8: external-attributes-LSA

Special area types

Stub area

Totally stubby area

NSSA (not-so-stubby-area)

NSSA totally stubby area