The Proxy SG has a minimum of 3 interfaces, even on the smallest 300 model. Let's take the SG 300-25 for our example. It has the following interfaces:
Two of these interfaces are usually connected by either a hardware or software bridge. In our above example interface 1:0 & 1:1 are on the same hardware bridge. Do not connect both interfaces in the bridge (1:0 & 1:1) to the same switch. Doing so will cause a switching loop. Instead, connect int 0:0 and either int 1:0 or 1:1 to the switch.