5.1 Sample Cases
* Single-homed site, single prefix
A separate AS is not needed; the prefix should be placed in an
AS of the provider. The site's prefix has exactly the same rout-
ing policy as the other customers of the site's service
provider, and there is no need to make any distinction in rout-
ing information.
This idea may at first seem slightly alien to some, but it high-
lights the clear distinction in the use of the AS number as a
representation of routing policy as opposed to some form of
administrative use.
In some situations, a single site, or piece of a site, may find
it necessary to have a policy different from that of its
provider, or the rest of the site. In such an instance, a sepa-
rate AS must be created for the affected prefixes. This situa-
tion is rare and should almost never happen. Very few stub sites
require different routing policies than their parents. Because
the AS is the unit of policy, however, this sometimes occurs.
* Single-homed site, multiple prefixes
Again, a separate AS is not needed; the prefixes should be
placed in an AS of the site's provider.
* Multi-homed site
Here multi-homed is taken to mean a prefix or group of prefixes
which connects to more than one service provider (i.e. more than
one AS with its own routing policy). It does not mean a network
multi-homed running an IGP for the purposes of resilience.
An AS is required; the site's prefixes should be part of a
single AS, distinct from the ASes of its service providers.
This allows the customer the ability to have a different repre-
sentation of policy and preference among the different service
providers.
This is ALMOST THE ONLY case where a network operator should
create its own AS number. In this case, the site should ensure
that it has the necessary facilities to run appropriate routing
protocols, such as BGP4.