Cisco ASA 5500 Failover

Scenario:
2 Data lines interface to our ASA firewall, and a Cisco Catalyst 2960. On the switch itself there are 3 VLANs for Data Line 1, Data L1, LAN. How can I configure the router if data line 1 fails, for data line 2 to automatically take over. If you need more clarity, just let me know.