Proxy ARP

Because Address Resolution Protocol (ARP) requests are MAC-layer broadcasts, they reach only the devices that are directly connected to the sender of the ARP request. ARP requests do not cross routers. However, when Proxy ARP is enabled on a device that is connected to two subnets, the device can respond to an ARP request from the other subnet.

For example:

The device is connected to the 10.10.10.0/24 and 10.20.20.0/24 subnets.
The device can respond to an ARP request from 10.10.10.69 for the MAC address of the device with the IP address 10.20.20.69.
In standard ARP, a request from a device in the 10.10.10.0/24 subnet cannot reach a device in the 10.20.20.0 subnet.

The ARP reply contains the device‘s MAC address instead of the MAC address of the target host. In this transaction, the traffic sent to the target host is forwarded through Layer 3 rather than being switched through Layer 2.

Note

Under some Layer 2 configurations, such as an uplink switch or private VLAN, broadcast packets are not flooded to every port in a VLAN. In these configurations, an ARP request from one host may not reach another host. When Proxy ARP is enabled locally on a port, the device is directed to reply on behalf of a target host if it exists.