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Traffic inside and outside an AS always
flows according to the road map laid out by routes. Altering the
routes, changes traffic behavior. Among the questions that
organizations and service providers ask about controlling routes are
these: 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 these questions and more.
When a BGP speaker receives updates
from multiple autonomous systems 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 or Local Preference) that the update contains and
other configurable BGP factors. The following sections describe
these key attributes that BGP uses in the decision-making process:
- Next Hop
- AS_Path
- Atomic Aggregate
- Aggregator
- Local Preference
- Weight
- Multiple Exit Discriminator (MED)
- Origin
Another BGP attribute, Community, is
considered in Chapter 9.
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