4. Common errors in allocating ASes

The term AS is often confused or even misused as a convenient way of grouping together a set of prefixes which belong under the same administrative umbrella, even if within that group of prefixes there are various different routing policies. Without exception, an AS must have only one routing policy.

It is essential that careful consideration and coordination be applied during the creation of an AS. Using an AS merely for the sake of having an AS is to be avoided, as is the worst-case scenario of one AS per classful network (the IDEAL situation is to have one prefix, containing many longer prefixes, per AS). This may mean that some re-engineering may be required in order to apply the criteria and guidelines for creation and allocation of an AS that we list below; nevertheless, doing so is probably the only way to implement the desired routing policy.

If you are currently engineering an AS, careful thought should be taken to register appropriately sized CIDR blocks with your registration authority in order to minimize the number of advertised prefixes from your AS. In the perfect world that number can, and should, be as low as one.

Some router implementations use an AS number as a form of tagging to identify interior as well as exterior routing processes. This tag does not need to be unique unless routing information is indeed exchanged with other ASes. See "IGP Issues".