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This document explains how to configure a default route, or gateway of last resort. These IP commands are used:

 

  • ip default-gateway

  • ip default-network

  • and ip route 0.0.0.0 0.0.0.0


ip default-gateway

The ip default-gateway command differs from the other two commands. It should only be used when ip routing is disabled on the Cisco router. Or it can be used on the Cisco switch.

For instance, if the router is a host in the IP world, you can use this command to define a default gateway for it. You might also use this command when your low end Cisco router is in boot mode in order to TFTP a Cisco IOS® Software image to the router. In boot mode, the router does not have ip routing enabled.

This example defines the router on IP address 172.16.15.4 as the default route:

ip default-gateway 172.16.15.4
ip default-network

Unlike the ip default-gateway command, you can use ip default-network when ip routing is enabled on the Cisco router. When you configure ip default-network the router considers routes to that network for installation as the gateway of last resort on the router.

For every network configured with ip default-network, if a router has a route to that network, that route is flagged as a candidate default route.

Note: The ip default-network command is classful. This means that if the router has a route to the subnet indicated by this command, it installs the route to the major net. At this point neither network has been flagged as the default network. The ip default-network command must be issued again, using the major net, in order to flag the candidate default route.

ip route 0.0.0.0 0.0.0.0

Creating a static route to network 0.0.0.0 0.0.0.0 is another way to set the gateway of last resort on a router. As with the ip default-network command, using the static route to 0.0.0.0 is not dependent on any routing protocols. However, ip routing must be enabled on the router.

Note: IGRP does not understand a route to 0.0.0.0. Therefore, it cannot propagate default routes created using theip route 0.0.0.0 0.0.0.0 command. Use the ip default-network command to have IGRP propagate a default route.

EIGRP propagates a route to network 0.0.0.0, but the static route must be redistributed into the routing protocol.

In earlier versions of RIP, the default route created using the ip route 0.0.0.0 0.0.0.0 was automatically advertised by RIP routers. In Cisco IOS Software Release 12.0T and later, RIP does not advertise the default route if the route is not learned via RIP. It may be necessary to redistribute the route into RIP.

The default routes created using the ip route 0.0.0.0 0.0.0.0 command are not propagated by OSPF and IS-IS. Additionally, this default cannot be redistributed into OSPF or IS-IS using the redistribute command. Use the default-information originate command to generate a default route into an IS-IS or OSPF routing domain. For more detailed information on behavior of default routes with OSPF, refer to How Does OSPF Generate Default Routes?