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Graphic: Route filter not
propagating routing updates
Thus far, you have learned a variety of routing protocols and how
they propagate routing information throughout an internetwork. There
are times, however, when you do not want routing information
propagated, as in the following examples:
- When using an on-demand WAN link
--- You may want to
minimize, or stop entirely, the exchange of routing update
information across this type of link; otherwise, the link will
remain up constantly.
When you want to prevent routing loops --- Many companies have
large enough networks where redundant paths are prominent. In some
cases, for example, when a path is learned to the same destination
by two different routing protocols, you may want to filter the
propagation of one of the paths.
When you want to preserve bandwidth --- Available bandwidth
becomes a precious resource as networks grow. You can ensure maximum
bandwidth availability for data traffic by reducing unnecessary
routing update traffic.
This section discusses the following ways you can control or
prevent routing update exchange and propagation:
--- Prevents all routing updates
from being sent through an interface. For EIGRP and OSPF, this
method includes hello protocol packets.
Default routes --- Instructs the router that if it does not
have a route for a given destination, to send the packet to the
default route.
Static routes --- A route to a destination that you configured
in the router. In contrast, dynamic routes are those learned via a
routing protocol such as RIP or Enhanced Interior Gateway Routing
Protocol (EIGRP).
Route update filtering --- Use access lists to filter route
update traffic about specific networks.
In order to make these capabilities effective, you must know your
network traffic patterns and know what the intended goal is when
applying the capabilities. If you do not know the problem you want
to resolve when using these capabilities, you will not know how to
verify that they are being effective.
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