OSPF Area & LSA types
OSPF area types and the LSAs they accept:
Area 0 (backbone) - LSA types 1, 2, 3, 4, 5
Non - backbone, non -stub - LSA types 1, 2, 3, 4, 5
Stub - LSA types 1, 2, 3
Totally Stub - LSA types 1, 2
Not - so -Stubby (NSSA) - LSA types 1, 2, 3, 4, 7
LSA types and descriptions:
* Type 1 (Router -LSA): Describe the collected states of the router's interfaces.
* Type 2 (Network - LSA): Describe the set of routers attached to the network.
* Type 3 or 4 (Summary - LSA): Describe inter - area routes, and enable the condensation of routing information at area borders. Originated by area border routers, type 3 summary -LSAs describe routes to networks, while type
4 summary -LSAs describe routes to AS boundary routers.
* Type 5 (AS - External -LSA): Describe routes to destinations external to the Autonomous System. Type 5 LSAs originate from AS boundary routers. A default route for the Autonomous System can also be described by an
AS - External - LSA.
* Type 7: Allow redistribution to occur between NSSA ’s. Since type 5 LSAs are not allowed in NSSA areas, the Autonomous System Boundary Router (ASBR) for the NSSA generates type 7 LSAs instead. These remains
within the NSSA and are translated into a type 5 ’ s by the NSSA ABR.
Inter -area routes generated by an ABR are called summary routes.
It is possible to configure summarized route to consolidate subnets. These summarized routes would be sent to standard and stub areas, but not totally stub areas.
Totally stubby areas will not accept Type - 3 or Type - 4 LSAs; Stub areas will (neither accepts type - 5). Totally stubby areas are Cisco proprietary; stub areas are not.
OSPF Area Types
These include:
* Standard - Accepts internal, external and summary LSA ’s.
* Backbone (transit area) - In multi -area OSPF networks all other areas must connect directly to this area in order to exchange route information. It must be labeled area “ 0 ”, and it accepts all LSA types. This behaves like a
normal Standard area, except it happens to reside in the middle of the network.
* Stub - Refers to an area that does not accept Type -5 LSAs to learn of external ASs. If routers need to route to networks outside the autonomous system, they must use a default route.
* Totally Stubby – All LSAs except Type 1 and 2 are blocked. Intra -area routes and the default route are the only routes passed within a totally stubby area. This is Cisco proprietary.
* Not - so -stubby (NSSA) – This is a non -proprietary extension of the existing stub area feature that allows the injection of external routes in a limited fashion into the stub area. Redistribution into an NSSA area creates a
special type of link -state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA, and an NSSA area border router (ABR)
translates it into a type 5 LSA, which gets propagated into the OSPF domain.
Just to make sure this sticks in your mind, here is a list of the OSPF area types and the LSAs they accept:
* Standard and Backbone (Area 0) - LSA types 1, 2, 3, 4, 5
* Non - backbone, non -stub - LSA types 1, 2, 3, 4, 5
* Stub - LSA types 1, 2, 3
* Totally Stub - LSA types 1, 2
* Not - so -Stubby (NSSA) - LSA types 1, 2, 7
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