3Com 2928 - Baseline Plus Switch PWR, User Manual

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1-8
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.