HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Services Conf - Page 9

Configuring ARP, Overview, ARP message format, ARP operation

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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 device uses ARP to resolve the IP address of the next hop to the corresponding MAC address. ARP message format ARP messages include 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 ARP message fields: • Hardware type-The hardware address type. 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, which describes type of the 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 operation If Host A and Host B are on the same subnet and Host A sends a packet to Host B, as shown in Figure 2, the resolution process is: 1

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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 device uses ARP to resolve the IP address of the next hop to the corresponding
MAC address.
ARP message format
ARP messages include 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
ARP message fields:
•
Hardware type
—The hardware address type. 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, which describes type of the 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 operation
If Host A and Host B are on the same subnet and Host A sends a packet to Host B, as shown in
Figure
2
, the resolution process is: