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Configuring IP Unicast Routing
Configuring Protocol-Independent Features
Specifying Default Routes and Networks
A router might not be able to learn the routes to all other networks. To provide complete routing capability, you can use
some routers as smart routers and give the remaining routers default routes to the smart router. (Smart routers have
routing table information for the entire internetwork.) These default routes can be dynamically learned or can be
configured in the individual routers. Most dynamic interior routing protocols include a mechanism for causing a smart
router to generate dynamic default information that is then forwarded to other routers.
If a router has a directly connected interface to the specified default network, the dynamic routing protocols running on
that device generate a default route. In RIP, it advertises the pseudonetwork 0.0.0.0.s
A router that is generating the default for a network also might need a default of its own. One way a router can generate
its own default is to specify a static route to the network 0.0.0.0 through the appropriate device.
When default information is passed through a dynamic routing protocol, no further configuration is required. The system
periodically scans its routing table to choose the optimal default network as its default route. In IGRP networks, there
might be several candidate networks for the system default. Cisco routers use administrative distance and metric
information to set the default route or the gateway of last resort.
If dynamic default information is not being passed to the system, candidates for the default route are specified with the
ip default-network global configuration command. If this network appears in the routing table from any source, it is
flagged as a possible choice for the default route. If the router has no interface on the default network, but does have a
path to it, the network is considered as a possible candidate, and the gateway to the best default path becomes the
gateway of last resort.
BEFORE YOU BEGIN
The ip default-network command is a classful command. It is effective only if the network mask of the network that you
wish to configure as a candidate route for computing the gateway of last resort matches the network mask in the Routing
Information Base (RIB).
For example, if you configure ip default-network 10.0.0.0, then the mask considered by the routing protocol is
10.0.0.0/8, as it is a Class A network. The gateway of last resort is set only if the RIB contains a 10.0.0.0/8 route.
If you need to use the ip default-network command, ensure that the RIB contains a network route that matches the major
mask of the network class.
DETAILED STEPS
Use the no ip default-network network number global configuration command to remove the route.
EXAMPLE
The following example defines a static route to network 10.0.0.0 as the static default route:
ip route 10.0.0.0 255.0.0.0 10.108.3.4
ip default-network 10.0.0.0
Command Purpose
1. configure terminal Enter global configuration mode.
2. ip default-network network number Specify a default network.
3. end Return to privileged EXEC mode.
4. show ip route Display the selected default route in the gateway of last
resort display.
5. copy running-config startup-config (Optional) Save your entries in the configuration file.
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