Configuring Route Maps and Policy Based Routing

Perhaps one of th e most colorful descriptions for route maps is that route maps are like duct tape for the network—not necessarily because they can be used to fix or mend something broken, but because they can be applied to numerous situations to address many issues. At times, they may not be th e most "pretty solutions," but they will be very effective. After you learn to configure and use route maps, you will soon see why some engineers refer to them as route tape. In policy-based routing (PBR), for instance, you may use a route map when traffic has to follow a particular path through the internetwork. This path may differ from the path the routing:) protocol wants to forward traffic on. PBR, along with route maps, enables the network engine ea to ^^smntially override the route table and influence which way traffic flows.

You also ca n apply route maps in a number of ways. The following list contains some of the more common and powerful applications of route maps:

• Route filtering during redistribution between routing protocols

• Route conerol and aft nbute modifi cation on BGP neighbors

• Route metric modification or tagging during redistribution between routing protocols

After you have route maps in your engineering tool kit, you will have one of the most powerful and versati le co nfigurati on options available on Cisco ro uters. This chapter discusses how to configure and use route maps and how to configure PBR.

Route Map Overview

Route maps are much like the "If . . . Then . . ." statements of many programming languages. If a condition is true, then do something. Route maps enable you to define routing policy that will be cc before the router examines its forwarding table; therefore, you can define routing policy that takes precedence overthe different route processes. This is why route maps are some of the most power commands you can use on a router. Example 2-1 highlights route map logic.

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