Markus,
What you are looking to do is definitely possible. The first step that you will want to do is configure HSRP on the interface going to your LAN. Take a look at the URL below for an example configuration.
http://www.cisco.com/warp/public/459/hsrp_bgp.html
In the example, one of the routers is tracking a serial interface. In your case, you need to also track the interface that your LL is connected to. One of the options in the configuration of tracking is to specify a value to decrement the routers standy priority. If you use the configuration from the example, the value is not specified but defaults to a value of 10. On the router that is doing the interface tracking, the priority has been set to a value of 105. This causes the router to become the active router within the HSRP group. (The second router in the example has had the priority value left at the default of 100.) What will happen when the LL interface goes down is that the active router will decrement its priority by a value of 10, giving it a new priority of 95. At this point, the standby router will assume the active role since it now has the higher priority.
To get the secondary router to bring up the ISDN link, you will need to have a routing protocol running between the two routers. The routing protocol needs to be configured to run on the subnet that connects the two routers to the LAN, and also needs to be connfigured to run on the subnet assigned to the LL interface. (You can stop routing protocol advertisements from going out the LL interface with the "passive-interface " command.)
With the routing protocol running between the two routers, you can now configure the secondary router to dial the ISDN link by configuring dialer-watch. When configuring dialer-watch, you will want to set it up to watch the route for the subnet that is assigned to the LL. When this subnet drops out of the routing table of the secondary router, it will bring up the ISDN connection. See the link below for more information on configuring dialer-watch.
http://www.cisco.com/warp/public/129/bri-backup-map-watch.html
Now that the ISDN connection is up and running and has assumed the active role within HSRP, you will need a way to get traffic to the remote site that has been dialed. This can be done either with a routing protocol, or you a static default route that points at the dialer interface. For example, "ip route 0.0.0.0 0.0.0.0 dialer1". Of course you will also need a way to get the return traffic from the dialed site back to the calling site. Again this can be done with a routing protocol or using a static route that has a higher administrative distance that the primary route. For the floating static route to work though the primary route would have to fall out of the routing table at the dialed site. Use of a routing protocol would definitely make this much easier to accomplish.
Steve
