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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Fundamentals and General Operation

BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)

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:

• No Transit Policy: An AS can have a policy that it will not handle transit traffic at all.
  
  
• Restricted AS Transit Policy: An AS may allow handling of traffic from certain ASes but not others. In this case it tells the ASes it will handle that they may send it traffic, but does not say this to the others.
  
  
• Criteria-Based Transit Policy: An AS may use a number of different criteria to decide whether to allow transit traffic. For example, it might allow transit traffic only during certain times, or only when it has enough spare capacity.

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.

BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)