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It is not feasible, or even desirable, for every
router to maintain routes to every possible destination. Instead, routers can
keep a default route, or a gateway of last resort. Default routes are used when
the router can not match a destination network with a more specific entry in the
routing table; thus, the gateway of last resort. In effect, the router uses the
default route to hand off to another router. The other router must have either a
route to that destination or its own default route to a third router. If it is a
default route to a third router, that router must have either the route to the
destination or another default route, and so on. Eventually, the packet
should be routed to a router that actually has a route to the destination.
A key scalability feature is that default routes keep routing tables as lean
as possible. They make it possible for routers to forward packets destined to
any Internet host without having to maintain a table entry for every Internet
network. Default routes can be statically entered by an administrator or
dynamically learned via a routing protocol.
Default routing begins with the administrator. Before routers can dynamically
exchange default information, an administrator must configure at least one
router with a default route. An administrator can use two very different
commands to statically configure default routes:
ip route 0.0.0.0 0.0.0.0
and
ip default-network.
The following sections explore these two methods in detail.
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