H3C Technologies H3C S12500-X Series Switches - Manual (Page 12)

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Configuring ARP
This chapter describes how to configure the Address Resolution Protocol (ARP).
Overview
ARP resolves IP addresses into MAC addresses on Ethernet networks.
ARP message format
ARP uses two types of messages: ARP request and ARP reply. Figure 1 shows the format of ARP
request/reply messages. Numbers in the figure refer to field lengths.
Figure 1 ARP message format
• Hardware type—Hardware address type. The value 1 represents Ethernet.
• Protocol type—Type of the protocol address to be mapped. The hexadecimal value 0x0800
represents IP.
• Hardware address length and protocol address length—Length, in bytes, of a hardware address
and a protocol address. For an Ethernet address, the value of the hardware address length field is
6. For an IPv4 address, the value of the protocol address length field is 4.
• OP—Operation code, which describes the type of ARP message. Value 1 represents an ARP request,
and value 2 represents an ARP reply.
• Sender hardware address—Hardware address of the device sending the message.
• Sender protocol address—Protocol address of the device sending the message.
• Target hardware address—Hardware address of the device to which the message is being sent.
• Target protocol address—Protocol address of the device to which the message is being sent.
ARP operating mechanism
As shown in Figure 2 , Host A and Host B are on the same subnet. Host A sends a packet to Host B as
follows:
1. Host A looks through the ARP table for an ARP entry for Host B. If one entry is found, Host A uses
the MAC address in the entry to encapsulate the IP packet into a data link layer frame. Then Host
A sends the frame to Host B.