Wired Intelligent Edge

I am trying to get a router connected to our firewall and would like to utilize subinterfaces for segmentation purposes. The firewall and router are both active/passive pairs. Since I would like communication to continue no matter what combination of devices is active, I want to connect both to a dedicated VLAN on my 5400r switch. Is there a way to accomplish this without an SVI? I tried connecting both devices to tagged/untagged ports on the switch and they could not communicate. 

EDIT: I must have had something configured wrong. Tried it again with the switch ports set as tagged and it worked fine. 

Depends on how exactly you configure the subinterface, but usually it means using an 802.1q tag. Accordingly, you need to use a tagged port or a trunk with tagging on the switch.