Deploying Policy Routing with Route Maps 99
Router A redistributes RIP into OSPF. Normally, the redistribution injects 16 routes, each with
a /24 mask, into area 0. Instead of doing that, Router A can summarize the 16 routes and inject
just a single route 172.20.16.0/20 into area 0. The following is Router A's OSPF conﬁguration:
router ospf 25
network 172.20.0.0 0.0.3.255 area 0
summary-address 172.20.16.0 255.255.240.0
where summary-address 172.20.16.0 255.255.240.0 is the key command that summarizes all
16 RIP routes into a single route 172.20.16.0/20. Router A then advertises this summary route
to area 0 so that the other OSPF backbone routers learn the route to the RIP destinations.
Deploying Policy Routing with Route Maps
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 bandwidth 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 with the access list syntax. Consult Chapter 6 as needed.