3Com Switch 8800 Advanced Software V5 Configuration Guide

198 CHAPTER 20: ARP CONFIGURATION
A device can implement the following functions by sending gratuitous ARP
packets:
■ Determining whether its IP address is already used by another device.
■ Informing other devices of its MAC address change so that they can update
their ARP entries.
A device receiving a gratuitous ARP packet can add the information carried in the
packet to its own dynamic ARP entry table if it finds no corresponding ARP entry
for the ARP packet in the cache.
Configuring Gratuitous
ARP
Follow these steps to configure gratuitous ARP:
Configuring ARP
Source Suppression
Introduction to ARP
Source Suppression
If hosts on a network attack the device by sending large amounts of IP packets
whose IP addresses cannot be resolved, the following consequences will be
resulted in:
■ The device sends large amounts of ARP request messages to the destination
subnet, which increases the load of the destination subnet.
■ The device continuously resolves destination IP addresses, which increase the
load of the CPU.
To protect a device against this kind of attack, Switch 8800s provide for the ARP
source suppression function. With the function enabled, whenever the number of
packets with unresolvable IP addresses that a host sends to the device within five
seconds exceeds the specified threshold, the device drops all subsequent packets
with the same source IP address in another five coming seconds. This helps in
protecting the device against the attack.
Configuring ARP Source
Suppression
To do... Use the command... Remarks
Enter system view system-view -
Enable the device to send
gratuitous ARP packets
gratuitous-arp-sending
enable
Optional
Disabled by default
Enable the gratuitous ARP
packet learning function
gratuitous-arp-learning
enable
Required
Disabled by default
To Do... Use the command... Remarks
Enter system view system-view -
Enable ARP source
suppression
arp source-suppression
enable
Required
Disabled by default
Set the maximum number of
packets with the same source
IP address but unresolvable
destination IP addresses that
the device can receive in five
seconds
arp source-suppression
limit limit-value
Optional
10 by default