This topic has been discussed several times in this post.
For this, you need to use a feature called reliable static routes using object tracking.
Using this you can configure the router to send periodic pings to the other end ip of the service provider. As long as this ping works fine, the defautl route that you have configured in the router will be taking effect. Once the router sees that no long the ping to the specified host is working, the default route in your router will be removed and the next best route available in your routing table ( floating static route pointing to the ISDN) will take effect.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...