OSPF Areas and Router Types

 Re-call that OSPF maintains an identical copy of the link state database on every router within an area by flooding LSA's. The maintenance of this database and flooding can sometimes overburden the CPU resources on routers, particularly when there are many routers in the OSPF area. To overcome this issue OSPF has the concept of areas. An OSPF area is just a logical grouping of OSPF routers, breaking one domain into smaller sub-domains, to reduce the flooding domain. Breaking up the single domain into smaller ones accomplishes a couple of things:

• Hides topology information for routers outside of the domain
• Reduces the impact on the routers memory as adding areas reduces the size of the database
• A smaller link state database also reduces the load on the CPU

Areas are identified with a 32-bit ID called the Area ID. The area can be expressed as either dotted decimal or a decimal number. Since there can be multiple areas, what do we call the routes from each area?

• Intra-Area Routes: These are routes that are shared between routers in the SAME area
• Inter-Area Routes: These are routes that are shared between routers in DIFFERENT areas
• External Routes: These are routes that are injected or imported INTO OSPF and shared between routers within the OSPF domain. 

Area 0(or 0.0.0.0) is the Area ID is usually reserved for the backbone. The Backbone Area is used to summarize the topologies of non-backbone areas to other non-backbone areas. All areas SHOULD attach to the backbone as that is how non-backbone areas exchange routes. There are five other area types besides the backbone. 

Three through six would be considered non-backbone areas also but these are different than standard areas. Standard non-backbone areas don't have restrictions on LSA's like the other area types. 

Non-Backbone Areas are standard areas other than area 0. In our example below, both Area 1 and Area 2 are non-backbone areas. These areas aren't special and will allow Type 3, 4, and 5 LSA's through.

Here is an example of what a multi-area topology might look like.

Before we go any further on areas let's talk about router types. There are four router types within OSPF. Note that a router CAN be classified by more than one type depending on the topology. 

• Internal Router: Internal routers are routers with all interfaces residing in the same area and have, as a result, a single link state database
• Area Border Router (ABR): ABR's are routers that 'border' two areas. They connect one or more areas to the backbone. These routers have separate link state databases for each area. The purpose of the ABR is to summarize the topology information of its areas into the backbone. 
• Backbone Router: A backbone router is just a router with at-least one interface in area 0, the backbone.
• Autonomous System Boundary Routers (ASBRs): An ASBR is a router that injects routes that were learned from outside of the OSPF domain. These routes could be from another IGP, EGP, or static, then redistributed into OSPF. The image below displays an ASBR router that is learning routes from EIGRP and then redistributing them into OSPF and vice versa

Stub Areas are the first special area. This area type blocks Type 5, the AS External LSA. The ABR at the edge of the area will advertise a default route (0.0.0.0) via a Type 3 Network Summary LSA into the area. This means that Stub Areas will only allow Inter-Area routes from the backbone as Type 3 LSA's and the single default route as a Type 3 LSA. The Stub routers accomplish this by setting the E-bit in their hello packets to 0. They will ignore hello packets with the E-bit set to 1 so every router in the stub area must have their E-bit set to 0 in order to form an adjacency. No routers within a stub area can be an ASBR, which makes sense, since we are blocking Type 5 LSA's from the area. 

Totally Stubby Areas go even further than stub areas. Totally Stubby Areas will not only block Type 5 LSA's, like in stub areas, but also block Type 3 LSA's. The one exception is that it will only allow a single Type 3 LSA, the default route. The default route will be how the routers reach not only external routes but also the inter-area routes from other areas.

Not-so-Stubby Areas are the same as Stub areas in that it blocks Type 5 External routes while allowing Type 3 inter-area routes. Where NSSA differs is when you have routes from another routing domain within the stub area. NSSA allows you to redistribute those routes outside of the domain where the stub area did not allow redistribution. The ASBR generates Type 7 LSA when it redistributes into OSPF. The ABR of the NSSA area will NOT allow Type 7 LSA's and will generate a Type 5 in its place to forward to the other areas.

Totally Not-so-Stubby Area (Totally NSSA) is the same as Totally stubby areas in that it blocks both Type 3 inter-area routes and Type 5 external routes. Where Totally NSSA differs is when you want to do redistribution from a routing domain outside of OSPF. Like NSSA the ASBR will generate Type 7 LSA's when you redistribute into OSPF. Again, like with NSSA the ABR of the Totally NSSA area will generate Type 5 LSA's to replace the Type 7 LSA. 

There is a lot of information here and there is still information that I did not cover. For more information I would recommend you read "TCP/IP Volume I" by Jeff Doyle. I hope this information made things clear, let me know in the comments what you think and what I missed.