7.4 The 0.0.0.0 Default Route
7.4.5 Using floating static routes
Floating static routes are static routes that are always in the configuration of a router but are installed in a routing table only when a dynamic route to the same network is lost. The dynamic route overrides the static route because the static route is configured with a greater distance.
Lab Activity
  In this lab, you will learn how to use a floating static route to setup a backup route that will only appear in the routing table when the link advertised by the routing protocol fails.

This type of static route can be used when the alternative link is a dialup connection. Dynamic routing information is usually sent or received over a dialup link only if the link is intended to be up for a long period of time. Remember that dynamic routing protocols send updates at regular intervals, causing the link to go up and down on a regular basis.

This scenario provides a real-life example of a floating static route being installed in a routing table after the lower-cost (shorter-path or lower-metric) routes are removed.

In the main figure, the link between router A and router C is now an ISDN connection. This link is used when the primary link between router A and router B goes down.

The partial configuration from router A shows how to configure a floating static route. The 130 is the distance parameter. It overrides the distance of 120 that RIP uses. (see RouterA#show running-config command output).

In the routing table from router A, you can see that router A has installed dynamic RIP routes for networks 10.0.0.0 and 0.0.0.0 via router B (see RouterA#show ip route (1) command output).

In the routing table from router A, you can see that router A has not received an update for networks 10.0.0.0 and 0.0.0.0 for almost two minutes. These routes will time out soon. (see RouterA#show ip route (2) command output).

In the output of the debug ip routing command from router A, you can see the old routes being removed and the new ones being accepted. Note the higher distances (130) in the new routes. (see RouterA#debug ip routing command output). 

In the routing table from router A, you can see the new routes installed. (see RouterA#show ip route (3) command output). 

The dial-on-demand routing (DDR) commands necessary to actually cause router A to make an ISDN call to router C have not been included. DDR is beyond the scope of this semester. The key factor needed for DDR to work is that the router must decide that the dial-up interface is the correct interface to use for forwarding a packet to its next hop.

In this scenario, router A has just installed routes that will cause it to forward packets destined to 10.1.0.0 via its ISDN interface. If we assume that the DDR commands are properly configured, the call should be made and a connection should be established.