Hi Robert,
better would be this message to be posted in the Security section .
As to your question you can have the FWSM with some interfaces configured with the standby IP address and some other without. However in the process of detecting if the mate is really down only the interfaces with the standby IP address will be used.
Moreover I expect the interfaces with no standby IP address not to swap the MAC addresses after the failover.
HTH
Alessandro
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