CISC-RT-000150 - The Cisco router must be configured to have Gratuitous ARP disabled on all external interfaces.
A gratuitous ARP is an ARP broadcast in which the source and destination MAC addresses are the same. It is used to inform the network about a host IP address. A spoofed gratuitous ARP message can cause network mapping information to be stored incorrectly, causing network malfunction.
Disable gratuitous ARP as shown in the example below: R5(config)#no ip gratuitous-arps