5. Criteria for the decision -- do I need an AS?
* Exchange of external routing information An AS must be used for exchanging external routing information with other ASes through an exterior routing protocol. The cur- rent recommended exterior routing protocol is BGP, the Border Gateway Protocol. However, the exchange of external routing information alone does not constitute the need for an AS. See "Sample Cases" below. * Many prefixes, one AS As a general rule, one should try to place as many prefixes as possible within a given AS, provided all of them conform to the same routing policy. * Unique routing policy An AS is only needed when you have a routing policy which is different from that of your border gateway peers. Here routing policy refers to how the rest of the Internet makes routing decisions based on information from your AS. See "Sample Cases" below to see exactly when this criteria will apply.
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.
5.2. Other factors
* Topology Routing policy decisions such as geography, AUP (Acceptable Use Policy) compliance and network topology can influence decisions of AS creation. However, all too often these are done without consideration of whether or not an AS is needed in terms of adding additional information for routing policy decisions by the rest of the Internet. Careful consideration should be taken when basing AS creation on these type of criteria. * Transition / "future-proofing" Often a site will be connected to a single service provider but has plans to connect to another at some point in the future. This is not enough of a reason to create an AS before you really need it. The AS number space is finite and the limited amount of re-engineering needed when you connect to another service provider should be considered as a natural step in transition. * History AS number application forms have historically made no reference to routing policy. All too often ASes have been created purely because it was seen as "part of the process" of connecting to the Internet. The document should be used as a reference from future application forms to show clearly when an AS is needed.