Understanding OSPF Areas

Concept of Areas is central to OSPF networks.

An OSPF network can be divided into sub-domains called areas. An OSPF area is a logical collection of OSPF networks, routers, and links (also virtual links) that have the same area identification parameter.

Every router within an OSPF area must maintain topological database for that area to which it belongs. The router doesn't have detailed information about network topology outside of its area, thereby reducing the size of its database.
Concept of OSPF areas allows controlling/limiting the scope of route information distribution. However, it is not possible to do route update filtering within an area.
Control of route information distribution is achieved using route summarization and route filtering between different OSPF areas, which reduces the number of routes to propagate.
The link-state database (LSDB) of routers within the same area must be synchronized and be exactly the same.
Following rules govern use of OSPF areas in a network:

• A backbone area   Each OSPF network must have a backbone area ‘0’, which combines a set of independent areas into a single domain.
• Each non-backbone area must be directly connected to the backbone area (direct connection or direct logical connection through a virtual link).
• The backbone area should be planned in a manner that it is not partitioned or divided into smaller pieces, under any (simple) failure conditions.

In proceeding OSPF Areas discussions terms of ABR, ASBR and external routes will be repeatedly referred, so a quick recap of both:

Area Border Router (ABR)

A router with interfaces in two (or more) different areas is an area border router. An area border router is in the OSPF boundary between two areas. Both sides of any link always belong to the same OSPF area.

Autonomous System Boundary Router (ASBR)

An ASBR advertises external destinations throughout the OSPF autonomous system. External routes are the routes redistributed into OSPF from any other protocol. In many cases, external link states make up a large percentage of the link states in the databases of every router.

External routes

It refer to updates passed from another routing protocol into the OSPF domain by the Autonomous System Border Router (ASBR).

Types of OSPF Areas

Normal Areas

These areas can either be standard areas or transit (backbone) areas. Standard areas are defined as areas that can accept intra-area, inter-area and external routes. The backbone area is the central area to which all other areas in OSPF connect.

Stub Area

These areas do not accept routes belonging to external autonomous systems (AS); however, these areas have inter-area and intra-area routes. In order to reach the outside networks, instead of using multiple external routes, a default summary route (0.0.0.0) is inserted into the stub area.
Stub areas are shielded from external routes but receive information about networks that belong to other areas of the same OSPF domain.
A stub area is typically configured in situations where the branch office need not know about all the routes to every other office, instead it could use a default route to the central office and get to other places from there. Hence the memory requirements of the leaf node routers are reduced, and so is the size of the OSPF database.
To define a stubby area, in Ciso IOS issue following command on ABR:

area <area id> stub

Totally Stub Areas

These areas do not allow routes other than intra-area and the default routes to be propagated within the area. The ABR injects a default route into the area and all the routers belonging to this area use the default route to send any traffic outside the area.
To define a total stubby area, in Ciso IOS issue following command on ABR:

area <area id> stub no-summary

Not So Stubby Area

NSSA 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, while retaining other stubby characteristics.
Assume that one of the routers in the stub area is connected to an external AS running a different routing protocol, it now becomes the ASBR, and hence the area can no more be called a stub area. However, if the area is configured as a NSSA, then the ASBR generates a NSSA external link-state advertisement (LSA Type-7) which can be flooded throughout the NSSA area only. These Type-7 LSAs are converted into Type-5 LSAs at the NSSA ABR and flooded throughout the OSPF domain.
Please note, it is a Type 7 LSA that is generated by an NSSA ASBR. Type 5 LSAs are not allowed in NSSA areas (due to its stubby natury), so the NSSA ASBR generates a type 7 LSA instead, which remains within the NSSA. This type 7 LSA gets translated back into a type 5 by the NSSA ABR.
To define an nssa, in Ciso IOS issue following command on each router in the area:

area <area id> nssa

There are two variants to NSSA.

• One is a simple NSSA as above, that blocks type 5 and type 4 LSAs, but allow type 3 LSAs.
• Second variant is NSSA totally stub areas, which allow only summary default routes and filters everything else.

To define an nssa totally stub area, in Ciso IOS issue following command on NSSA ABR only:

area <area id> nssa no-summary

All other routers in the area will be configured as previously:

area <area id> nssa

Filtering in NSSA

There are situations where there is no need to inject external routes into the NSSA as type 7. This situation usually occurs when an ASBR is also an NSSA ABR. You can prevent the router from creating type 7 LSAs for NSSA with this command:

Area <area id> nssa no-redistribution