3Com Switch 7750 Configuration Guide Guide

Introduction to ARP 549
Introduction to
Gratuitous ARP
The following are the characteristics of gratuitous ARP packets:
■ Both source and destination IP addresses carried in a gratuitous ARP packet are
the local addresses, and the source MAC address carried in it is the local MAC
addresses.
■ If a device finds that the IP addresses carried in a received gratuitous packet
conflict with those of its own, it returns an ARP response to the sending device
to notify of the IP address conflict.
By sending gratuitous ARP packets, a network device can:
■ Determine whether or not IP address conflicts exist between it and other
network devices.
■ Trigger other network devices to update its hardware address stored in their
caches.
When the gratuitous ARP packet learning function is enabled on a switch and the
switch receives a gratuitous ARP packet, the switch updates the existing ARP entry
(contained in the cache of the switch) that matches the received gratuitous ARP
packet using the hardware address of the sender carried in the gratuitous ARP
packet. A switch operates like this whenever it receives a gratuitous ARP packet.
Gratuitous ARP Update
Interval
Overview of gratuitous ARP update interval
When ARP aging timer expires, some hosts in the network directly delete the ARP
entries learned dynamically, incapable of updating ARP entries actively. These hosts
have to trigger a new ARP request packet with a new IP packet received to request
for the gateway address. As a host can buffer only one packet, when a ping is sent
with a long packet, multiple fragments will be lost, which interrupts the ping.
When network load or the CPU occupancy of the receiving host is high, ARP
packets may be lost or the host may be unable to process the ARP received timely.
In such a case, after the dynamic ARP entries on the host age out, the traffic
between the host and the sending device will remain interrupted before the host
learns the ARP entries on the sending device again.
To address this issue, you can configure the gratuitous ARP update interval on the
Switch 7750 Family. With gratuitous ARP packets sent periodically, the receiving
host can update the ARP entry for the gateway in its ARP table timely. In this way,
the ARP entry for the gateway has been updated before the host ages out the
entry; therefore, this entry will not be deleted. This prevents traffic interruption as
mentioned above.
How gratuitous ARP update interval works
A switch periodically sends gratuitous ARP packets that carry the master IP address
and secondary IP address of VLAN interfaces and the virtual IP addresses of all the
VRRP backup groups to update the ARP entries on the device that is connected to
the switch and incapable of updating ARP entries actively.
If a small number of VLAN interfaces and VRRP backup groups are configured, it
takes a very time for the device to traverse all the VLAN interfaces and their IP
addresses. If the traffic loops without being limited, gratuitous ARP packets are
sent to the same IP address at an interval too short. This increases switch work