| Traffic inside and outside an AS
always flows according to the road map laid out by routes. Altering
the routes translates to changes in traffic behavior. Among the
questions that organizations and service providers ask about
controlling routes are: How do I prevent my private networks from
being advertised? How do I filter routing updates coming from a
particular neighbor? How do I make sure that I use this link or this
provider rather than another one?
Through the use of
attributes, BGP provides the answer to all of these questions and
more.
When a BGP speaker receives updates
from multiple ASs that describe different paths to the same
destination, it must choose the single best path for reaching that
destination. Once chosen, BGP propagates the best path to its
neighbors. The decision is based on the value of attributes (such as
NEXT_HOP, Administrative Weights, Local Preference, the Origin of
the Route, and Path Length) that the update contains and other BGP-configurable
factors. This section describes the following attributes and factors
that BGP uses in the decision-making process:
- AS_path
- Origin
- NEXT_HOP
- Weight
- Local Preference
- MULTI_EXIT_DISC (MED)
- Community
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