110 Chapter 3: Managing Routing Protocols
By default, packets that are generated by the router are not policy routed. To change this, issue
the global command ip local policy route-map, like this:
RTA(config)#ip local policy route-map TESTMAP
where the keyword TESTMAP in the preceding command identiﬁes the route map to use for
policy routing locally generated packets.
As you extend, enhance, and optimize the capabilities of your network, you must deal with
the crucial function of routing. This chapter covered some important routing services that help
you manage network scalability, complexity, and stability. These services (and your mastery
of them) directly impact the number of users and locations you can support, your ability to
integrate new networks and topologies, and the stability of the network as it expands.
The following are the key concepts of this chapter:
• A passive interface is an interface on which you deliberately suppress the advertising of
routing updates. Use the passive-interface command to make an interface passive.
• Route ﬁlters give you granular control over the routes sent and received by your router. To
ﬁlter routes, use access lists and the distribute-list router conﬁg mode command.
• Redistribution is a service that allows the sharing of routes across different routing
domains—for example, between RIP and OSPF. It transfers routes between distinct
domains and makes connectivity possible across them. Enable this service with the
redistribute router conﬁg mode command.
• When redistributing routes into OSPF, don't forget to use the subnets keyword if you need
to redistribute both major nets and their subnets.
• The administrative distance is a priority assigned to a route based on its routing protocol.
A route with a lower administrative distance is preferred over a route with a higher
• To prevent route feedback caused by mutual redistribution, you should always control
redistribution with route ﬁlters.
• Although classful routing protocols do not support VLSM, sometimes you are forced to
redistribute from a classless domain into a classful one, and routes might be lost in the
process. As a workaround, you might be able to originate and redistribute static routes that
are agreeable to the classful domain.
• A default route is a special route that tells the router how to reach unknown destinations.
How you originate and propagate a default route depends on the routing protocol.
• Be careful when you conﬁgure a default route on a RIP router. RIP automatically
advertises the default route out all RIP-enabled interfaces. Watch out for accidental
propagation of default routes on the network.
• When you use a default route to reach a subnet of a connected major net, you must ensure
that the router is conﬁgured with the ip classless global conﬁg command.
• EIGRP and OSPF support route summarization: the consolidation of multiple contiguous
routes into a single generalized route to promote efﬁcient and stable routing.
• EIGRP automatically summarizes across major net boundaries. When you deploy
discontiguous subnets, you might have to disable EIGRP auto-summarization.
• Use the ip summary-address interface command to conﬁgure EIGRP route
• To summarize OSPF routes between areas, use the area range router conﬁg mode
command. To summarize routes as they are redistributed into OSPF, use the summary-
address router conﬁg mode command.
• Policy routing allows you to override normal forwarding decisions with route maps. Route
maps are built of one or more entries that are created with the route-map global conﬁg
command. Each entry contains match and set statements that deﬁne the routing policy.
• You can also use policy routing to set IP precedence and classify packets for QoS.