In FWSM the standby IP address is used to for failover. To set the IP address of the failover interface, enter the following command:

primary(config)# failover interface ip failover_interface ip_address mask standby

ip_address

The standby IP address must be in the same subnet as the active IP address. You do not need to identify the standby address subnet mask.

Note this setting because this command is the same on the secondary module.

The failover link IP address and MAC address do not change at failover. The active IP address always stays with the primary module, while the standby IP address stays with the secondary module.

http://www.cisco.com/en/US/docs/security/fwsm/fwsm23/configuration/guide/failover.html#wpxref34096