Chapter 14. Working with Routing Policies
Your router (or other L3 device) gathers location information from routing protocols and stores it in its routing tables. These routing tables are then whittled down to include only the best information, and that information is stored in the forwarding table, which is ultimately responsible for where traffic is forwarded.
With all of that in place, what if the information being learned by the routing protocols are wrong? You're receiving information from your neighbors, and what if your neighbors are doing something you do not want them to do? You need a way of controlling how information is imported into your router. And on the flip side of the coin, you need a way to prevent your router from doing things that neighboring routers might not like. Routing policies are that way. Routing policies are your solution to monitoring, filtering, or even modifying what gets into and what comes out of your routing and forwarding tables.
Constructing Routing Policies
Routing protocols allow routers to exchange information and figure out where everyone 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. And from that repository of routing knowledge, your router selects only the best routes. These best routes form the forwarding table ...