8.7 The Routing Process
8.7.6
How to control BGP routes with BGP attributes
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