The most basic option is a floating static route: A static route with a higher Admin Distance than the default of 1.
Let me explain. Let's say you have two interfaces: Fa0/0 with IP 1.1.1.1/30 (uplink IP is .2), and the Dialer0 interface that handles dial-up. To use the Ethernet link, you have configured a static default gateway as such:
ip route 0.0.0.0 0.0.0.0 1.1.1.2
A floating static route would be added as follows:
ip route 0.0.0.0 0.0.0.0 Dialer0 $AdminDistance$
where $AdminDistance$ is any value between 1 and 255.
The way this works is as follows: Any static route, by default, have an AD of 0. So a static route would always be the first choice for a router to insert into the routing table, provided that the destination IP is valid. When the Ethernet interface goes down, the route to 1.1.1.2 is removed, and the route to Dialer0 is installed in the routing table. If Fa0/0 come back up, it's route is preffered (lowest AD) and the route to Dialer0 is removed.
There is one catch: The destination IP (1.1.1.2) bust not be available in the routing table for the default route to be removed. For this to happen, either the port must physically go down (a bit of a problem if you connect via L2 devices), or you need to track the destination IP (most easily done by implementing PPP).
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by: ikalmarPosted on 2009-10-16 at 00:17:33ID: 25587494
Hi,
In this case the IP SLA good solution... why don't you wanna implement it?
Best regards,
Istvan