Connected: An Internet Encyclopedia
5. Policy Making with BGP
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5. Policy Making with BGP
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:
- 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.)
- 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.)
- 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:
- An AS can minimize the number of transit AS's. (Shorter AS
paths can be preferred over longer ones.)
- 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.
- 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.
Next: 6. Path Selection with BGP
Connected: An Internet Encyclopedia
5. Policy Making with BGP