Policy routing enables you to direct traffic over user-defined paths based on the flexible syntax of access lists. With policy routing, you use enhanced filters called route maps to override normal forwarding decisions like those based on dynamic routing protocols. Route maps contain your criteria for identifying traffic and your instructions on how that traffic should be forwarded. You might want to do this to support certain routing policies, such as these:
• You want different applications (Web, e-mail. Telnet) to travel over different paths. That is, you want some applications to travel over normal paths determined by routing protocols, but you need other applications to travel over alternate paths—perhaps for performance or bandwiddi allocation reasons.
• For legal, contractual, or security reasons, certain types of traffic must go over a different path than other types of traffic.
• You need to assign links to different groups of people for billing purposes. Each group uses and pays for its own bandwidth pipe.
In addition to directing traffic over different paths, policy routing enables you to set IP precedence values in packets. This marks (or classifies) packets with a certain quality of service (QoS) level that queuing and discarding services might use to prioritize traffic in the network. Another IOS service, Committed Access Rate (CAR), also classifies packets by setting IP precedence values. CAR and other advanced QoS features are covered in Chapter 5, "Deploying Advanced Quality of Service Features." Chapter 4, "Deploying Basic Quality of Service Features," covers IP precedence, QoS concepts, and basic QoS features.
This section covers both forms of policy routing: forwarding traffic over user-defined paths and classifying traffic with IP precedence.
NOTE This section requires familiarity widi die access list syntax. Consult Chapter 6 as needed.
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