BGP/Theory

BGP is designed for large scale networks, and this fact has great influence on it.

The main concept of BGP is automonous system. The idea is: different networks in an internet do not need to know the internal structure of each other, they only need to know how to reach each other. Such information as IGP, connections between routers and many other things is important inside a network, but it wouldn't help other networks to reach it. So we make our network look as something integer, identified by its autonomous system number.

On the other hand, it would be helpful to know the detailed information about how to reach some network, since most part of autonomous systems are not directly connected and traffic from one to another often goes through several of them. So BGP features "AS path" which is set of ASN traffic need to go to reach its destination.