Each ISP modem is connected to a Layer 2 unmanaged switch, which then one port is connected to one FW the other the other FW.
Different switches per WAN correct?
Each box is identical, except one is Master and the other Backup of course so I know my HA sync is working.
The SYNC interface has nothing to do with the CARP VIP status on each interface or which node is master or backup at any given time.
https://forum.netgate.com/post/719523
My problem here is when I have one ISP connected the IP address assigned to the VIP never shows up on the modems ARP table.
The CARP MAC only shows up in the upstream MAC address table due to the CARP advertisements.
When the node holding the CARP MASTER status sees an ARP request for the CARP VIP, it answers with an ARP response. This ARP response is sourced from the interface MAC address but contains the CARP MAC address as the ISAT MAC address.
There is no reason for the modem to contain the CARP VIP in its ARP table unless it needs to route traffic from itself to the CARP address.
That said, MANY ISP devices simply do not do what is necessary for CARP to function correctly. They might only allow one MAC per port or any of a number of silly things.
Some work fine.