The two appliances in the high availability pair are installed onto the same subnet in either a parallel arrangement or a one-arm arrangement, both of which are shown in the following figure. In a one-arm arrangement, use the apA.2 port (and, optionally, the apB.2 port), not the apA.1 port. Some models require a separate management LAN, whether deployed in inline or one-armed mode. This is depicted only in the middle diagram.
Figure 1. Cabling for High-Availability Pairs
Do not break the above topology with additional switches. Random switch arrangements are not supported. Each of the switches must be either a single, monolithic switch, a single logical switch, or part of the same chassis.
If the spanning-tree protocol (STP) is enabled on the router or switch ports attached to the appliances, failover will work, but the failover time may increase to roughly thirty seconds. Without STP, failover time is roughly five seconds. Thus, to achieve the briefest possible failover interval, disable STP on the ports connecting to the appliances.
Figure 2. Ethernet Port Locations (Older Models)