HA with single WAN mac/ip

[mcouture]

I would like to setup 2 FWs with carp or otherwise to run in HA mode mainly to support for upgrades.  iE: patch the backup then move primary to backup and patch the master.    CARP should cover this, however I only have access to 1 "authorized" MAC address/IP address.  Will I have to tell my ISP the CARP MAC address is the only authorized MAC address?