Looking Ahead

Mastering routing at the edges of your domain gives you full control over traffic into and out of your autonomous system. Another piece of the puzzle is how the traffic flows inside the AS before it gets out. Not all routers inside the AS run BGP. IGP-only routers usually do not carry a full list of Internet routes due to memory constraints. Running defaults inside the AS to reach external routes is one of the most common ways for internal routers to reach destinations outside the AS. With defaults comes the threat of routing loops if conflicting policies exist between your BGP and your IGP. The following chapter discusses these issues of how to make BGP policies flow hand-in-hand with IGP defaults. That chapter also discusses the use of policy routing in achieving total control over routing behaviors based on the sources of IP addresses rather than the traditional destination-based routing.