Introduction To Gratuitous Arp - Huawei Quidway S3100 Series Operation Manual

Operation Manual – ARP
Quidway S3100 Series Ethernet Switches
Suppose there are two hosts on the same network segment: Host A and Host B.
The IP address of Host A is IP_A and that of Host B is IP_B. To send a packet to
Host B, Host A checks its own ARP mapping table first to see if the ARP entry
corresponding to IP_B exists. If yes, Host A encapsulates the IP packet into a
frame with the MAC address of Host B inserted to it and sends it to Host B.
If the corresponding MAC address is not found in the ARP mapping table, Host A
adds the packet in the transmission queue, creates an ARP request packet and
broadcasts it throughout the Ethernet. As mentioned earlier, the ARP request
packet contains the IP address of Host B, the IP address of Host A, and the MAC
address of Host A. Since the ARP request packet is broadcasted, all hosts on the
network segment can receive it. However, only the requested host (namely, Host
B) processes the request.
Host B appends the IP address and the MAC address carried in the request
packet (that is, the IP address and the MAC address of the sender, Host A) to its
ARP mapping table and then sends a ARP reply packet to the sender (Host A),
with its MAC address inserted to the packet. Note that the ARP reply packet is a
unicast packet instead of a broadcasted packet.
Upon receiving the ARP reply packet, Host A extracts the IP address and the
corresponding MAC address of Host B from the packet, adds them to its ARP
mapping table, and then transmits all the packets in the queue with their
destination being Host B.
Normally, ARP performs address resolution automatically, without the intervention of
the administrator.

1.1.5 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
Huawei Technologies Proprietary
1-4
Chapter 1 ARP Configuration