Chapter 9. Controlling Large-Scale Autonomous Systems

This chapter covers the following key topics:

• Route reflectors— A method of managing expanding mesh requirements in large autonomous systems (ASs) by using selected routers as focal points for internal BGP sessions.

• Confederations— A method of managing expanding mesh requirements in large ASs by creating sub-ASs.

• Controlling IGP expansion— Methods of managing networks in which expansion is characterized by the use of multiple IGPs.

Autonomous systems consisting of hundreds of routing nodes can pose a serious routing management problem to network administrators. Service providers and customers each have their own set of problems when dealing with large networks. On the service provider side, the majority of routers run Border Gateway Protocol (BGP). Because of the BGP rule that states that one Interior Border Gateway Protocol (IBGP) speaker can't advertise a route learned from another IBGP speaker to a third IBGP speaker, the IBGP mesh can quickly grow beyond the provider's control. On the customer side, however, the majority of routers run Interior Gateway Protocols (IGPs), which also might grow beyond the customer's control.

This chapter discusses methods and techniques that can be used to better control the deployment of BGP and IGPs inside large autonomous systems. There are no absolute rules that say a provider or customer should or should not use one of the methods discussed in this chapter, or which method is best. Keep in mind that any new technique brings with it its own complexities. Imposing complex techniques on situations that do not really need them could hurt more than help, but in contrast, planning ahead can save you a considerable number of headaches down the road.