IP Rules

One of the topics that you have not seen is the supposed original basis for using Policy Routing in the first place—the ability to route based on source, TOS, packet data, and other packet features. This is where the final member of the Policy Routing triad, rules, enters the scene.

As you saw in Chapter 3, rules are what provide the decision structure in the RPDB. Rules function not just as logical packet selectors, but also possess the capability to act upon a selected packet. In this sense the true power becomes apparent. Rules have much the same set of actions as routes when acting directly on a packet. Unlike routes, they cannot specify any forwarding actions but only the blocking actions. To illustrate, consider Example 5.7 ...

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