Redistribution of Routing Protocols
359
Router A and Router B are core level routers. Router C is a small access
router. The potential problem is that Router A could learn about network
10.1.2.0
via Router C instead of Router B, if no distribute lists are used to
control what routes are advertised from Router C.
If all of Router A's traffic destined for 10.1.2.0 were routed through
Router C, it could easily overwhelm the small router. In this scenario, we
only want Router C to have redundant links to the core, and not let the core
transit an access router to reach another core router.
The problem can be solved or avoided by configuring an access list that
permits only networks connected to Router C. The access list would be
applied outbound to the interfaces connecting Routers A and B with the
distribute-list
command. The command is issued from within the rout-
ing protocol configuration mode.
Distribute lists can solve problems as well as cause problems. If the down-
stream routers are configured to learn their default gateway dynamically, the
router must have the default network in the route table. If the route is not
present, the router will lose the gateway of last resort. When a distribute list
is applied, you must verify that it allows route advertisement of the default
network, as well as any other crucial routes.
Route Maps
Route maps are used to manipulate routing. They are small scripts that can
contain multiple instances and multiple conditions for each instance. Route
maps are somewhat like access lists if you specify that the packet must match
an access list. In addition to the capability of permitting or denying the
packet, you can define what is done before the packet is forwarded.
Route maps can be used to set metrics for route updates, set a command
to its default value, and so on. Table 6.13 gives a list of what a route map
can do.
T A B L E 6 . 1 3
Route Map Configuration Commands
Command
Description
default
Set a command to its defaults.
exit
Exit from route-map configuration mode.
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