Constructing Routing Policies
Routing protocols allow routers to exchange information and figure out where every device in the network is located. When a routing protocol like BGP shares information about the BGP routes it knows, these routes are received by your router and stored in the routing table. The routing table represents the collective knowledge of all the routing information that your router has (IGPs such as OSPF contribute their share of information, too). And from that repository of routing knowledge, your router selects only the best routes. These best routes are used as the forwarding table, which provides the final word on all forwarding decisions.
As shown in Chapter 10, the best route to link-state protocols such as OSPF and IS-IS might be the one with the fewest hops, or the one with the most bandwidth, or the one that follows a multicast topology, and so on. BGP has a bewildering array of attributes that you can tweak to influence the AS path that a packet takes through a large network. This chapter is all about controlling the flow of this routing information, especially for BGP.
Routing policies are the constructs designed by administrators to control what goes into and what comes out of the routing table. That is, routing policies act as an inbound filter into the routing table and ultimately determine what information your router shares with other routers through outbound router advertisements. (See Chapter 10 for more information on router advertisements.) ...