Area
An OSPF area is a smaller part of the larger OSPF AS. Areas are used to limit the link-state updates that are sent out. The flooding used for these updates would overwhelm a large network, so it is divided into these smaller areas for manageability.
Within an area if there are two or more routers that are viable, there will always be a designated router (DR) and a backup DR (BDR). For more on these router roles, see
“Designated router (DR) and backup router (BDR)”.
Defining a private OSPF area, involves:
• assigning a 32-bit number to the area that is unique on your network
• defining the characteristics of one or more OSPF areas
• creating associations between the OSPF areas that you defined and the local networks to include in the OSPF area
• if required, adjusting the settings of OSPF-enabled interfaces.
| IPv6 OSPF area numbers use the same 32-bit number notation as IPv4 OSPF. |
If you are using the web-based manager to perform these tasks, follow the procedures summarized below.
FortiGate units support the four main types of OSPF area:
Backbone area
Every OSPF network has at least one AS, and every OSPF network has a backbone area. The backbone is the main area, or possibly the only area. All other OSPF areas are connected to a backbone area. This means if two areas want to pass routing information back and forth, that routing information will go through the backbone on its way between those areas. For this reason the backbone not only has to connect to all other areas in the network, but also be uninterrupted to be able to pass traffic to all points of the network.
The backbone area is referred to as area 0 because it has an IP address of 0.0.0.0.
Stub area
A stub area is an OSPF area that receives no outside routes advertised into it, and all routing in it is based on a default route. This essentially isolates it from outside areas.
Stub areas are useful for small networks that are part of a larger organization, especially if the networking equipment can’t handle routing large amounts of traffic passing through, or there are other reasons to prevent outside traffic, such as security. For example most organizations don’t want their accounting department to be the center of their network with everyone’s traffic passing through there. It would increase the security risks, slow down their network, and it generally doesn’t make sense.
A variation on the stub area is the totally stubby area. It is a stub area that does not allow summarized routes.
NSSA
A not-so-stubby-area (NSSA) is a stub area that allows for external routes to be injected into it. While it still does not allow routes from external areas, it is not limited to only using he default route for internal routing.
Regular area
A regular area is what all the other ASes are, all the non-backbone, non-stub, non-NSSA areas. A regular area generally has a connection to the backbone, does receive advertisements of outside routes, and does not have an area number of 0.0.0.0.