8.1 Autonomous Systems
8.1.2 Single-homed autonomous systems
If an AS has only one exit point to outside networks, it is considered a single-homed system. Single-homed autonomous systems are often referred to as stub networks or stubs. Stubs can rely on a default route to handle all traffic destined for non-local networks. For the network depicted in Figure , you can configure routers in the customer AS to use a default route to an upstream service provider.

However, how will the outside world learn about the network addresses inside the customer's AS? The provider can essentially use three different methods to advertise a customer's networks.

  • Use a static configuration. The provider could list the customer's networks as static entries in its own router and then advertise these routes upstream to the Internet core. This approach works well if the customer's networks can be summarized using a CIDR prefix, as discussed in Chapter 2. However, if the AS contains numerous discontiguous networks, route aggregation may not be a viable option.
  • Use an IGP. Another alternative is to use an IGP to advertise the customer's networks, as depicted in Figure . This has all the benefits of dynamic routing, in which network information and changes are dynamically sent to the provider.
  • Use an EGP. The third method by which the ISP can learn and advertise the customer's routes is to use an EGP such as BGP. It is difficult to get a registered AS number from IANA for a stub network 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 AS numbers (64,512 to 65,535), and strip off these numbers when advertising the customer's routes towards the core of the Internet (Figure ).

Note that only one of these three solutions requires a customer to run BGP with its provider.