Common proxy ARP
A common proxy ARP enabled device allows hosts that reside on different subnets to communicate.
As shown in Figure 6, Switch connects to two subnets through VLAN-interface 1 and VLAN-interface 2. The IP addresses of the two interfaces are 192.168.10.99/24 and 192.168.20.99/24. Host A and Host B are assigned the same prefix 192.168.0.0. Host A connects to VLAN-interface 1 and Host B connects to VLAN-interface 2.
Figure 6: Application environment of common proxy ARP
Because Host A and Host B have the same prefix 192.168.0.0, Host A considers that Host B is on the same network, and it broadcasts an ARP request for the MAC address of Host B. However, Host B cannot receive this request because it is in a different broadcast domain.
You can common enable proxy ARP on VLAN-interface 1 of the switch so that the switch can reply to the ARP request from Host A with the MAC address of VLAN-interface 1, and forward packets sent from Host A to Host B. In this case, the switch acts as a proxy of Host B.
A main advantage of common proxy ARP is that you can enable it on a single switch without disturbing routing tables of other routers in the network. Proxy ARP acts as the gateway for hosts that are not configured with a default gateway or do not have routing capability.