Bridge is something like a switch. So if machines are in same VLAN / IP subnet, then you want ports bridged/switched. If machines are in different VLANs / IP subnets, then you can't do anything else than routing and for that ports have to be treated as separate interfaces, each having own IP address from different subnets. And keep in mid that your CRS is a switch. It can route, but at low speeds.
Your explanation lacks details, so it's not possible to give you better feedback.
I think I have my answer. but with more details the machines are part of differents vlans/subnets like this:
VLAN 10: 192.168.10.2
VLAN 20: 192.168.20.2
VLAN 10: 192.168.10.3
VLAN 20: 192.168.20.3
VLAN 10: 192.168.10.4
VLAN 20: 192.168.20.4
VLAN 10: 192.168.10.5
VLAN 10 = ISCSI and A, B, C open a session to D
VLAN 20 is a metadata network, where A, B, C exchange their metadata regurlarly.
So If I have 4 machines in the same subnet / vlan for iscsi I will have to put them in a bridge, I guess. Which is what I do right now, but I was looking at an alternative to optimise it possibly :)