The first thing you have to understand about BGP policy is that it's a two-way street. The question of which of your connections you choose for outbound traffic is completely independent from the question of which connection does the Internet choose for your inbound traffic. Some of the same mechanisms are used to answer these questions, but they're still entirely independent questions.
From a load sharing point of view, one of these questions will dominate and the other will be largely insignificant for most ASes. For example, an AS doing a lot of web hosting, running well-known FTP servers, or providing streaming multimedia content will likely generate a lot more outbound traffic than it will receive inbound traffic. In these ASes, "which exit from the AS should I choose" is the key question because one or more exits may be running at or near full capacity much of the time.
In contrast, an AS that hosts little content but that instead provides Internet connectivity for many "surfers" may see the key load balancing question as "which entrance do I want the Internet to choose in delivering my incoming traffic?" They're more likely to have one or more of their inbound connections running at or near full capacity much of the time.
To summarize, one question is "which exit from your AS will you choose for outbound traffic?" The other is "which entrance will the Internet choose to deliver your inbound traffic?" Your role in answering these questions varies depending on who's asking the question and how many ISPs you have. But before we talk about controlling how the questions are answered, you should understand what's likely to happen when you do start exerting control.
Copyright © 1999-2000 by Robert A. Van Valzah