Multi-area OSPF allows OSPF to scale to fit considerably larger networks than Single Area OSPF can effectively service. In a multi-area OSPF configuration, routers can be grouped into four distinct catagories.

Router Type	Description
Internal Router	All router interfaces are in a single area
Backbone Router	At least one interface is in area 0
Area Border Router (ABR)	This router is attached to multiple areas
Autonomous System Boundary Router (ASBR)	At least one interface is connected to another autonomous system

Additionally, there are a total of five different area types for Cisco routers

Area Type	Description
Standard	A standard area accepts link updates and route summaries. This is the same as the area covered in Single Area OSPF
Backbone (transit)	This is area 0. All other areas must connect to this, as it conveys all information between areas.
Stub Area	A stub area doesn't accept routes to other autonomous systems. Instead it uses a default route (0.0.0.0/0)
Totally Stubby	Like the stub area, totally stubby areas don't accept routes to other autonomous systems. Additionally, totally stubbies don't accept routes to other areas. A default route is used instead (0.0.0.0/0). Totally Stubby is a Cisco proprietary area.
Not So Stubby (NSSA)	A totally stubby area is a special kind of stub that exists only with an ASBR. This allows importing of external routes via type 7 LSAs, which are then translated to type 5 at the ABR.

Link State Advertisements (LSAs) are used to update link-state tables. An LSA is encapsulated in a type 4 OSPF packet (LSU) as covered in Single Area OSPF. There are a total of 7 different LSAs as follows.

LSA Type	Abbreviation	Description
1. Router Link Entry	(O-OSPF)	This is generated by all routers for routers within their area. This is used within a single area only.
2. Network Link Entry	(O-OSPF)	This is generated by a DR for their specific, multiaccess area. This is used within a single area only.
3. Summary Link Entry	(IA-OSPF)	This is generated by an ABR and sent to the backbone and other ABRs. This describes the links internal to an ABR's specific area. This is used between areas only.
4. Summary Link Entry	(IA-OSPF)	This is generated by an ABR and sent to the backbone and other ABRs. This describes how to reach ASBRs located within the ABR's area. These are not sent to totally stubby areas. Inter-area only.
5. Autonomous System External Link Entry	(E1-OSPF external type 1)(E2-OSPF external type 2)	This is sent from an ASBR to an OSPF autonomous system describing external routes. These updates are not allowed in stub, totally stubby, or NSSA areas. E1 is for multiple routes to an external entity (multihomed AS) and the cost = external + internal cost. E2 is for only one external route (singlehomed AS) and the cost = external cost. E2 is the default.
6. Multicast OSPF	(MOSF)	This is not implemented by Cisco. It allows multicast distribution trees to forward multicast packets.
7. Autonomous System External Link Entry	(N1-OSPF NSSA type 1)(N2-OSPF NSSA type 2)	These LSAs are only sent by an ASBR that is part of a Not So Stubby area (NSSA). It is then translated to type 5 LSAs at the ABR before continueing through the OSPF autonomous system (AS).

When calculating its routing table, an multiarea OSPF router goes through several stages to find best routes. These are summarized below.

Step	Area	Description	LSAs used
1.	Local Area	Calculate all paths within own area.	Type 1 and Type 2
2.	Inter-Area	Calculate all paths to other areas.	Type 3 and Type 4 (not totally stubby)
3.	External	Calculate all paths to external ASes.	Type 5 or 7 (not stub)