H3C Technologies H3C S3100 Series Switches - Manual (Page 371)

Operation Manual – ARP
H3C S3100-52P Ethernet Switch Chapter 1 ARP Configuration
1-4
1.1.4 ARP Implementation Procedure
The ARP mapping table of a host is empty when the host is just started up. And when a
dynamic ARP mapping entry is not in use for a specified period of time, it is removed
from the ARP mapping table so as to save the memory space and shorten the interval
for the switch to look up entries in the ARP mapping table. For details, refer to Figure
1-3.
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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.
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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.
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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.
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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.