The Different Types of Areas

The only obligatory area is Area 0, also known as the backbone area or Area 0.0.0.0. In addition to the backbone area, which connects the other areas, OSPF networks use several other types of areas. The following are the different types of areas:

• An ordinary or standard area—This area, described earlier, connects to the backbone. The area is seen as an entity unto itself. Every router knows about every network in the area, and each router has the same topological database. However, the routing tables will be unique from the perspective of the router and its position within the area.

• A stub area—This is an area that will not accept external summary routes. The LSAs blocked are Types 4 (summary link LSAs that are generated by the ABRs) and 5. The consequence is that the only way that a router within the area can see outside the autonomous system is via the configuration of a default route. Every router within the area can see every network within the area and the networks (summarized or not) within other areas. It is typically used in a hub-and-spoke network design.

• A totally stubby area—This area does not accept summary LSAs from the other areas or the external summary LSAs from outside the autonomous system. The LSAs blocked are Types 3, 4, and 5. The only way out of the area is via a configured default route. A default route is indicated as via 0.0.0.0. This type of area is particularly useful for remote sites that have few networks and limited connectivity with the rest of the network. This is a proprietary solution offered only by Cisco. Cisco recommends this solution if you have a totally Cisco shop because it keeps the topological databases and routing tables as small as possible.

• A not so stubby area (NSSA)—This area is used primarily to connect to ISPs, or when redistribution is required. In most respects, it is the same as the stub area. External routes are not propagated into or out of the area. It does not allow Type 4 or Type 5 LSAs. This seems contradictory to the opening sentence of this paragraph, which stated that this type of area was used to connect to an ISP or for redistribution, both of which are external routes to OSPF. It is contradictory, and this area designed for the exception. Possible examples are an area with a few stub networks but with a connection to a router that runs only RIP, or an area with its own connection to an Internet resource needed only by a certain division.

To create an area that is seen as a stub area but that can receive external routes that it will not propagate into the backbone area, and thus the rest of the OSPF domain, involves creating a NSSA. Another LSA, Type 7, is created for the NSSA. This LSA may be originated and communicated throughout the area, but it will not be propagated into other areas, including Area 0. If the information is to be propagated into throughout the AS, it is translated into an LSA Type 5 at the NSSA ABR.

It is not always possible to design the network and determine where redistribution is to occur. RFC 1587 deals with this subject.

• The backbone area—This area is often referred to as Area 0, and it connects all the other areas. It can propagate all the LSAs except for LSA Type 7, which would have been translated into LSA Type 5 by the ABR.

Some restrictions govern creating a stub or totally stubby area. These restrictions are in place because no external routes are allowed in these areas:

• No external routes are allowed.

• No virtual links are allowed.

• No redistribution is allowed.

• No ASBR routers are allowed.

• The area is not the backbone area.

• All the routers are configured to be stub routers.

Now that you understand many components of OSPF, it is important to focus on some of the design implications of creating multiple areas. This focus will reinforce the concepts detailed in the chapter.

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