You need to take into consideration that the two units each have a presence on the network. The active unit uses the system IP address and the MAC addresses of the Primary unit. The Primary Unit is determined by the unit that has the end of the failover cable marked "Primary" plugged into it. The standby unit uses the failover IP address and the MAC addresses of the Secondary unit. If a switchover occurs, the units swap the IP address and MAC addresses they are using so as to replace each other's presence on the network. This action is invisible to the network.