3Com Switch 7750 Configuration Guide Guide

548 CHAPTER 52: ARP CONFIGURATION
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 saves 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 back an ARP reply packet to the sender (Host A),
with its MAC address carried in 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.
Figure 143 ARP work flow
Once ARP is deployed, the ARP work flow is automatically processed.