Understanding Autonomous Systems
An Autonomous System (AS) is a group of areas sharing a common routing strategy. For OSPF purposes, every Autonomous System (AS) must be assigned a unique 16-bit number by the Internet Assigned Numbers Authority (IANA).
The actual routing of information within an Autonomous System takes place in one of three ways:
These different types of routing will be discussed later under the section, Routing Within a Hierarchical Structure.
A typical scenario for many networks as they grow and more sites are added is that the benefits of OSPF begin to degrade. For example, the link-state database will continue to grow in size as the number of routers grows. At some point it will become inefficient. The flooding LSAs from a large number of routers can also cause congestion problems. To solve these problems, you begin by segmenting your Autonomous System (AS) into multiple areas. As you group routers into areas, consider limiting the number of routers per area. Each router will then have a link-state database with entries for each router in its area.
Areas are similar to the idea of a subnet in that the routes and networks contained within can be easily summarized. In other words, areas are contiguous logical segments of the network that have been grouped together. Through the use of areas within OSPF, the network will be easier to manage and will provide a marked reduction in routing traffic. These benefits are gained because the actual topology of an area is invisible to other routers outside of the area.
Areas also allow the routers contained within them to run their own link-state database and SPF algorithm. In truth, a router will run one copy of the link-state database for each area to which it is connected.
Characteristics of an OSPF Area
The following list provides some general characteristics of an OSPF area.
The characteristics outlined in the preceding list need to be considered when working within an OSPF network.
Area Design Rules
When designing an OSPF area, you should keep some of the following requirements in mind:
The Backbone Area
A backbone area is the logical and physical structure for the Autonomous System and is attached to multiple areas. The backbone area is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous, but it need not be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links, which will be discussed in Chapter 5, The Fundamentals of OSPF Routing & Design.
An area could be referred to as a stub area when there is a single exit point from that area, or if external routing to outside of the area does not have to take an optimal path. A stub is just what it sounds like, a dead end within the network. Packets can only enter and leave through the Area Border Router. A rather unique occurrence I will grant you, so why would you ever need such an area? The reason is that same old nuisance that keeps coming aroundnetwork size. By building stub areas, you can reduce the overall size of the tables within the routers that are inside the stub area.
External networks, such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area.
Configuring a stub area reduces the link-state database size inside an area and reduces the memory requirements of routers inside that area.
Routing from these areas to the outside world is based on a default route. They do contain inter-area and intra-area routes.
Stub areas should have one Area Border router.
All OSPF routers inside a stub area have to be configured as stub routers because whenever an area is configured as stub, all interfaces that belong to that area will start exchanging hello packets with a flag that indicates that the interface is stub. Actually this is just a bit in the hello packet (E bit) that gets set to 0. All routers that have a common segment have to agree on that flag. If the routers dont agree, then they will not become neighbors and routing will not take effect.
Stub Area Restrictions
Stub areas have certain restrictions applied to their operation. This is because they have been designed not to carry external routes and any of the situations in the following list could cause external links to be injected into the stub area.