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.
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