A default route is a route that is used when the routing table has no entry for the destination network in a packet that it is attempting to forward.
In larger networks, there may well be many static routes to be configured. Not only is this a chore for the administrator, but it also requires vigilance so that changes in the routing table can be reconfigured. It may be that turning on a routing protocol is advised, or, alternatively you can configure a specialized static route, called a static default route. To configure a default route, use the following syntax:
ip default-network network-number
This command will generate a default route to be sent in updates. It does not generate a default network on the router that was configured.
This is more obviously a static default route and will generate a default route on the router configured.
NOTE The different TCP/IP protocols treat these commands differently when redistributing them into the routing protocol. Reference the CCO documentation set for detailed explanations.
These default routes are propagated through the network dynamically or can be configured into the individual routers.
If a router has a directly connected interface onto the specified default network, the dynamic routing protocols running on that router will generate or source a default route. In the case of RIP, it will advertise the pseudonetwork 0.0.0.0. In the case of IGRP, the network itself is advertised and flagged as an exterior route.
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