Transparent Firewall with 2 redundant Hosts

[andre.paiz]

Hello PFSense people.

I'm implementing the scenario below:

1 Layer 3 switch that performs OSPF connectivity with my ISP.
1 Switch core that users the layer 3 switch as default gateway. The switch core connects all distribution switches from my internal network.
1 Transparent PFSense firewall between them.

I use transparent firewall with bridge because my network contains only valid IP addresses, and I don't think is necessary to route traffic twice. My layer 3 switch on the border is a robust equipment and I want to eliminate OSPF from the firewall.

I want to implement a secondary firewall host, to make them redundants in the case of a failure. In the future, I will also duplicate the L3 switches with a stack module, since I have only one connection with the ISP (for now).

How should I implement this 2 transparent firewalls and make them redundant with no human intervention? I have seen a lot of alerts on using CARP with this scenario, but something has to be configured, or it will create a loop on the network. Is Spanning Tree the best solution to implement redundancy? How about the State Table of traffic?

It is possible to implement what I'm wanting to do?

Thanks everyone.