Arp Process - 3Com MSR 50 Series Configuration Manual

550 C 30: ARP C
HAPTER ONFIGURATION

ARP Process

Hardware address length: Length of a hardware address in bytes. For an
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Ethernet address, the value of the hardware address length field is "6".
Protocol address length: Length of a protocol address, in bytes. For an IP(v4)
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address, the value of the protocol address length field is "4".
OP: Operation code. This field specifies the type of ARP message. For example,
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the value "1" represents an ARP request, "2" represents an ARP reply, and "4"
represents a reverse ARP request.
Sender hardware address: This field specifies the hardware address of the
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device sending the message.
Sender protocol address: This field specifies the protocol address of the device
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sending the message.
Target hardware address: This field specifies the hardware address of the device
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the message is being sent to.
Target protocol address: This field specifies the protocol address of the device
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the message is being sent to.
Suppose that Host A and Host B are on the same subnet and that Host A sends a
message to Host B, as shown in
1 Host A looks in its ARP mapping table to see whether there is an ARP entry for
Host B. If Host A finds it, Host A uses the MAC address in the entry to encapsulate
the IP packet into a data link layer frame and sends the frame to Host B.
2 If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an
ARP request, in which the source IP address and source MAC address are
respectively the IP address and MAC address of Host A and the destination IP
address and MAC address are respectively the IP address of Host B and an all-zero
MAC address. Because the ARP request is sent in broadcast mode, all hosts on this
subnet can receive the request, but only the requested host (namely, Host B) will
process the request.
3 Host B compares its own IP address with the destination IP address in the ARP
request. If they are the same, Host B saves the source IP address and source MAC
address into its ARP mapping table, encapsulates its MAC address into an ARP
reply, and unicasts the reply to Host A.
4 After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP
mapping table for subsequent packet forwarding. Meanwhile, Host A
encapsulates the IP packet and sends it out.
Figure 156. The resolution process is as follows: