Introduction To Gratuitous Arp - H3C S7500 Series Operation Manual

Operation Manual – ARP
H3C S7500 Series Ethernet Switches
Generally, a host will automatically trigger ARP procedure during IP addressing.

1.1.5 Introduction to Gratuitous ARP

Gratuitous ARP packets have the following characteristics:
Both source and destination IP addresses of a gratuitous ARP packet are the local
addresses; the source MAC address of it is the local MAC address.
If a device finds that the IP address in an incoming gratuitous packet conflicts with
its own IP address, it returns an ARP replay to the sending device to notify the
sender of the IP address conflict.
By sending gratuitous ARP packets, a network device can:
Determine whether or not IP address conflict exists between it and other network
devices.
Trigger other network devices to update its hardware address stored in their
caches.
With gratuitous ARP learning enabled on a device, each time the device receives a
gratuitous ARP packet, the device updates the ARP entry matching the packet in the
cache (if exists) by using the hardware address of the sender carried in the gratuitous
ARP packet.
I. 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
S7500 series Ethernet switches. 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.
II. 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 IP addresses of all the VRRP
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Chapter 1 ARP Configuration