8.1 Autonomous Systems
8.1.4 Stub autonomous system
An AS is considered stub, or single-homed, when it reaches networks outside its domain via a single exit point. Figure illustrates a single-homed or stub AS. A single-homed AS does not need to learn all the routes in the internetwork backbone, because there is only one way out -- all traffic destined to a network not within the AS can be routed along the default route toward this single exit point.

To connect to the Internet, an AS (or domain) will be connected to an Internet Service Provider (ISP), which then connects to the Internet backbone. Single-homed ASs on the Internet then have a single connection to an ISP. The service provider this single-homed AS is connected to must still provide information to the rest of the Internet about destinations which are reachable within its downstream network.

There are different methods for the provider to advertise the customer's routes. For instance, the provider could list the customer's subnets as static entries in its router and advertise these static entries towards the core of the network. This method scales well if the customer's routes can be represented by a small set of aggregate routes. When the customer has too many discontiguous subnets, listing all these subnets as static routes in the provider's routers becomes inefficient. Another alternative is to use an IGP to advertise the customer's networks towards the service provider. This has all the benefits of dynamic routing, where network information and changes are dynamically sent to the provider.

The third method by which the ISP can learn and advertise the customer's routes is to use BGP. It is difficult to get a registered AS number from IANA because the customer's routing policies are an extension of the policies of the provider. Instead, the provider can give the customer an AS number from the private pool of ASs (65412-65535), and strip these ASs out of the routing path information when advertising the customer's routes towards the core of the internetwork.

Quite a few combinations of protocols can be used between the ISP and the customer; Figure illustrates some of the possible configurations for stub ASs (external BGP [EBGP] and internal BGP [IBGP] are discussed in later sections). The service providers might place a customer router in their point of presence (POPs), or they might place their router in the customer's network. Note that not every situation requires a customer to run BGP with its provider, as mentioned earlier.