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| 9.1 |  | Autonomous Systems |
| 9.1.2 |  | 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 shown in Figure , configure the routers in the customer AS to use a default route to an upstream service provider.

The following are three methods to advertise a customer's networks from the provider's perspective:

- Use a static configuration – The provider lists the customer's networks as static entries in its own router and advertises 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 Module 2. However, if the AS contains numerous discontiguous networks, route aggregation may not be a viable option.
- Use an IGP – Both the provider and the customer use an IGP to share information regarding the customer's networks. This provides the benefits associated with dynamic routing.
- 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. In a single-homed autonomous system the customer's routing policies are an extension of the policies of the provider. For this reason the Internet number registries are unlikely to assign an AS number. Instead, the provider can give the customer an AS number from the private pool of AS numbers, 64,512 to 65,535. The provider will strip off these numbers when advertising the customer's routes towards the core of the Internet.

Notice that only the final solution requires the customers to enable BGP on their router.