Strict ARP Learning

If many user hosts simultaneously send a large number of ARP packets to a device, or attackers send bogus ARP packets to the device, the following problems occur:

Processing ARP packets consumes considerable CPU resources. The device learns many invalid ARP entries, which exhaust ARP entry resources and prevent the device from learning ARP entries for ARP packets from authorized users. Consequently, communication of authorized users is interrupted.
After receiving bogus ARP packets, the device incorrectly modifies the ARP entries. As a result, authorized users cannot communicate with each other.

To avoid the preceding problems, configure the strict ARP learning function on the gateway.

After the strict ARP learning function is enabled, the device learns only ARP entries for ARP reply packets in response to ARP Request packets that it has sent. In this way, the device can defend against most ARP attacks.

Figure 1 Strict ARP learning

As shown in Figure 1, after receiving an ARP Request packet from UserA, the gateway sends an ARP Reply packet to UserA and adds or updates an ARP entry matching UserA. After the strict ARP learning function is enabled on the gateway:

The gateway does not add or update an ARP entry for userA when it receives an ARP Request packet from UserA. If the ARP Request packet requests the MAC address of the gateway, the gateway sends an ARP Reply packet to UserA.
If the gateway sends an ARP Request packet to UserB, the gateway adds or updates an ARP entry matching UserB after it receives the ARP Reply packet.