H3C Technologies H3C S12500 Series Switches - Manual (Page 15)

1
Configuring ARP
Overview
The Address Resolution Protocol (ARP) is used to resolve an IP address into a physical address (Ethernet
MAC address, for example).
In an Ethernet LAN, a switch uses ARP to translate the IP address of the next hop to the corresponding
MAC address.
ARP message format
ARP messages are classified into ARP requests and ARP replies. Figure 1 shows the format of the ARP
request/reply. Numbers in the figure refer to field lengths.
Figure 1 ARP message format
An ARP message contains the following fields:
• Hardware type—The hardware address type. The value 1 represents Ethernet.
• Protocol type—The 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. The type of ARP message. The value 1 represents an ARP request and 2
represents an ARP reply.
• Sender hardware address—Hardware address of the switch sending the message.
• Sender protocol address—Protocol address of the switch sending the message.
• Target hardware address—Hardware address of the switch the message is being sent to.
• Target protocol address—Protocol address of the switch the message is being sent to.
ARP address resolution process
If Host A and Host B are on the same subnet and Host A sends a packet to Host B:
28-byte ARP request/reply
OP
Sender hardware
address
Sender protocol
address
Target hardware
address
Target protocol
address
Protocol
type
2 2 6 1 4 4 2 6 1
Hardware address length
Protocol address length
Hardware
type