3Com 2928 User Guide - Page 347

z Select the Advanced Options checkbox. z Type 100 for VLAN ID. z Select GigabitEthernet1/0/1 for Port. z Click Apply to complete the configuration. Gratuitous ARP Introduction to Gratuitous ARP In a gratuitous ARP packet, the sender IP address and the target IP address are both the IP address of the device issuing the packet, the sender MAC address is the MAC address of the device, and the target MAC address is the broadcast address ff:ff:ff:ff:ff:ff. A device implements the following functions by sending gratuitous ARP packets: z Determining whether its IP address is already used by another device. z Informing other devices about the change of its MAC address so that they can update their ARP entries. A device receiving a gratuitous ARP packet adds the information carried in the packet to its own dynamic ARP table if it finds no corresponding ARP entry exists in the cache. An attacker sends spoofed gratuitous ARP packets to hosts on a network. As a result, traffic that the hosts want to send to the gateway is sent to the attacker instead, and the hosts cannot access external networks. To prevent such gateway spoofing attacks, you can enable the gateway to send gratuitous ARP packets periodically. In this way, each host can learn correct gateway address information. Configuring Gratuitous ARP Select Network > ARP Management from the navigation tree, and click the Gratuitous ARP tab to enter the page shown in Figure 1-11. Figure 1-11 Gratuitous ARP configuration page Table 1-2 describes the gratuitous ARP configuration items. 1-8