|
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.
|