OSPF Area types

Quick area type overview

Unless specified otherwise, all OSPF area starts out as a standard area. Of course, area 0 is always the backbone and needs the correct details. This means that the type of area 0 is always normal. A normal network will always reatin all information. The information just gets aggregated at area borders1)This assumes you aren’t doing any manual summarisation..

All other network types are a method to remove information you don’t care about:
Making an area stubby removes all information external to the OSPF domain,
Making an area not-so-stubby does the same, but retains the possibility to inject external routing data within the area
Making a stub area or an NSSA into their total counterparts, also removes details from other OSPF areas. If we put it all into a matrix, we get this:

Type normal stub total stub NSSA total NSSA
Details from other area’s Y Y N Y N
External from other area’s Y N N N N
Can inject routes Y N N Y Y
Inject default route standard N Y Y N N

The role of the ABR

The “total” part of the area types, only has meaning on any router that knows information from other OSPF area’s. So, only ABRs have to care if an area is totally stubby or just stubby. I think of these area’s as consisting of three major types (normal, stub, NSSA) with two subtypes (total or not total). Neighborships will form when routers agree on the major area type.

Here we have our first chance to try to break something: What happens if we have two ABRs, one thinks the area is a stub, the other thinks the area is a total stub? Turns out, not much. Adjacencies comes up.
The total stub ABR only advertises a default route, the stub ABR advertises a default and some specific routes. You can see in the route output below that a router internal to the area (R1) sees two equal cost default routes, through the total stub ABR (R3, 10.1.3.3) and through the the normal stub ABR (R2, 10.1.2.2). This does mean that R3’s loopback (3.3.3.3) gets routed through R2.

The total stub router doesn’t complain about receiving any summary LSA’s:

In short
The purpose of all versions of stub area types are a type of filtering. The only difference is the amount of filtering. The stub vs NSSA version must match within an area for neighborships to come up. The addition of “Total” only matters on the ABR and tells that ABR whether or not to advertise OSPF internal routes.

References   [ + ]

1. This assumes you aren’t doing any manual summarisation.

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