Routing - ip default-network command

On a Cisco IOS router, the ip default-network command can be issued in the global configuration mode to indicate a default route for packets addressed to networks not explicitly listed in the Routing Table.

When the ip default-network command is used, 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.

When used in conjunction with routing protocols such as OSPF or EIGRP, the default route it creates can be advertised. For example when used with EIGRP, it can be used to flag the network as a default route so it can be redistributed.

Gateways of last resort selected via the ip default-networkcommand are propagated differently based on which routing protocol is used. For OSPF or IS-IS, default route created using the ip default-networkcommand is not propagated at all.

For EIGRP the command can be tricky to get it to be redistributed, as the following must be taken into account:

  1. The ip default-network command is classful, so if the router has a route to the subnet indicated by this command, it installs the route to the major network.
  2. In this event, once applied, neither network has been flagged as the default route. The ip default-network command must be run again, this time with the use of the major network in order to flag the candidate as a default route.

Because of its classful nature, and its general behavior, the command has been deprecated and should not typically be used in a production network. A preferred method is to use either the ip default-gateway command or the ip route static route routing table entry.