IP address pool

I used to assign multiple IP addresses to two NIC(ip alias) as IP address pool(dual connections) and use PF as firewall, but some times the addresses of one ISP will be lost(always one ISP, the other was fine, never lost), need restart network services and reapply PF settings.

I am not sure if the NIC bonding issue or ISP's problem related to ip address, so I am wondering, are there any way I can set like address pool instead of setup multiple ip on physical NIC on BSD system? like cisco firewall appliance to set ip address pool.
You can only lose your IP address if you use DHCP. Statically assigned addresses always 'stick' to the interface. Unless you destroy the interface.

Cisco's set ip address pool is used for the DHCP service it provides.