3Com Switch 7750 Configuration Guide

Introduction to ARP 573
ARP Implementation The ARP table of a host is empty when the host just starts up. When a dynamic
ARP entry has not been used for a specific time period, it is removed from the ARP
table. The purpose of this is to save memory space and update the entries in the
ARP table. The following describes the implementation procedure of ARP.
■ Suppose Host A and Host B are on the same network segment. 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
first looks up its own ARP table for an ARP entry that contains IP_B. If such an
entry is found, Host A encapsulates the IP packet into a frame by using the
MAC address in the entry as the destination MAC address and then sends the
frame to Host B.
■ If no such an entry is found in the ARP table, Host A puts the packet into the
transmission queue, generates an ARP request packet with the IP addresses of
Host B and Host A and the MAC address of Host A (IP_B, IP_A and MAC_A),
and broadcasts the request on the Ethernet. Since the ARP request is
broadcasted, all hosts on the network segment receive it. However, only the
host with IP address IP_B (Host B) will process the request further.
■ Host B adds the sender IP address and MAC address carried in the request (IP_A
and MAC_A of Host A) in an entry to its ARP table and then returns an ARP
reply packet to the sender (Host A), with its MAC address carried in the packet.
Note that the ARP reply is a unicast packet instead of a broadcasted packet.
■ Upon receiving the ARP reply, Host A extracts the IP address and MAC address
of Host B from the packet, adds them in an entry to its ARP table, and then
sends out all the packets destined for Host B in the transmission queue.
Generally, a host will automatically trigger ARP procedure during IP addressing.
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.
Type Entry type, which can be:
■ 1: Not any of the following
■ 2: Invalid entry
■ 3: Dynamic entry
■ 4: Static entry
Table 446 Field descriptions of an ARP table
Field Description