5. Policy Making with BGP

BGP provides the capability for enforcing policies based on various routing preferences and constraints. Policies are not directly encoded in the protocol. Rather, policies are provided to BGP in the form of configuration information.

BGP enforces policies by affecting the selection of paths from multiple alternatives and by controlling the redistribution of routing information. Policies are determined by the AS administration.

Routing policies are related to political, security, or economic considerations. For example, if an AS is unwilling to carry traffic to another AS, it can enforce a policy prohibiting this. The following are examples of routing policies that can be enforced with the use of BGP:

1. A multihomed AS can refuse to act as a transit AS for other AS's. (It does so by only advertising routes to destinations internal to the AS.)

2. A multihomed AS can become a transit AS for a restricted set of adjacent AS's, i.e., some, but not all, AS's can use the multihomed AS as a transit AS. (It does so by advertising its routing information to this set of AS's.)

3. An AS can favor or disfavor the use of certain AS's for carrying transit traffic from itself.

A number of performance-related criteria can be controlled with the use of BGP:

1. An AS can minimize the number of transit AS's. (Shorter AS paths can be preferred over longer ones.)

2. The quality of transit AS's. If an AS determines that two or more AS paths can be used to reach a given destination, that AS can use a variety of means to decide which of the candidate AS paths it will use. The quality of an AS can be measured by such things as diameter, link speed, capacity, tendency to become congested, and quality of operation. Information about these qualities might be determined by means other than BGP.

3. Preference of internal routes over external routes.

For consistency within an AS, equal cost paths, resulting from combinations of policies and/or normal route selection procedures, must be resolved in a consistent fashion.

Fundamental to BGP is the rule that an AS advertises to its neighboring AS's only those routes that it uses. This rule reflects the "hop-by-hop" routing paradigm generally used by the current Internet.