9.3 Route Selection Criteria

Generally speaking, additional rules for comparing routes among several alternatives are outside the scope of this document. There are two exceptions:

• If the local AS appears in the AS path of the new route being considered, then that new route cannot be viewed as better than any other route. If such a route were ever used, a routing loop would result.

• In order to achieve successful distributed operation, only routes with a likelihood of stability can be chosen. Thus, an AS must avoid using unstable routes, and it must not make rapid spontaneous changes to its choice of route. Quantifying the terms "unstable" and "rapid" in the previous sentence will require experience, but the principle is clear.