BGP Autonomous System Types, Traffic Flows and Routing Policies
(Page 2 of 3)
BGP Autonomous System Routing Policies
The reason why BGP makes a distinction between traffic types and AS types is the same as why it is done on the streets: many folks have a dim view of through traffic. In a neighborhood, everyone wants to be able to get from their homes to anywhere they need to go in the city, but they don't want lots of other people using their streets. Similarly, every autonomous system must use at least one other autonomous system to communicate with distance ASes, but many are less than enthusiastic about being a conduit for lots of external traffic.
This reluctance really does make sense in many cases, both in the case of a neighborhood or in the case of BGP. Having lots of cars and trucks on a residential street can be a problem in a number of ways: safety issues, wear and tear on the road, pollution and so forth. Similarly, if a multihomed AS was forced to carry all transit traffic that other ASes want to send to it, it might become overloaded.
To provide control over the carrying of transit traffic, BGP allows an AS to set up and use routing policies. These are sets of rules that govern how an AS will handle transit traffic. A great deal of flexibility exists in how an AS decides to handle transit traffic. Some of the many options include:
In a similar manner, policies can be set that control how an AS will have its own traffic handled by other autonomous systems. Obviously a stub AS will always connect to the internetwork as a whole using the single AS to which it connects. A multihomed AS, however, may have policies that influence route selection by specifying the conditions under which one AS should be used over another. These policies may be based on considerations of security (if one connecting AS is deemed more secure than another), performance (one AS is faster than another), reliability or other factors.
Home - Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.