H3c-technologies H3C S5120 Series Switches User Manual download pdf (Page 223)

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z Sender protocol address: This field specifies the protocol address of the device sending the

message.

z Target hardware address: This field specifies the hardware address of the device the message is

being sent to.

z Target protocol address: This field specifies the protocol address of the device the message is

being sent to.

ARP Operation

Suppose that Host A and Host B are on the same subnet and Host A sends a packet to Host B, as

shown in Figure 1-2

. The resolution process is as follows:

1) Host A looks into its ARP table to see whether there is an ARP entry for Host B. If yes, 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 sender IP address and the sender MAC address are the IP address and the MAC

address of Host A respectively, and the target IP address and the target MAC address are the IP

address of Host B and an all-zero MAC address respectively. Because the ARP request is a

broadcast, all hosts on this subnet can receive the request, but only the requested host (namely,

Host B) will respond to 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 in its ARP 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 to its ARP table. Meanwhile,

Host A encapsulates the IP packet and sends it out.

Figure 1-2 ARP address resolution process

If Host A is not on the same subnet with Host B, Host A first sends an ARP request to the gateway. The

target IP address in the ARP request is the IP address of the gateway. After obtaining the MAC address

of the gateway from an ARP reply, Host A sends the packet to the gateway. If the gateway maintains the

ARP entry of Host B, it forwards the packet to Host B directly; if not, it broadcasts an ARP request, in

which the target IP address is the IP address of Host B. After obtaining the MAC address of Host B, the

gateway sends the packet to Host B.

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