H3C Technologies H3C S12500-X Series Switches - Manual (Page 13)

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2. If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request. The
payload of the ARP request comprises the following information:
{
Sender IP address and sender MAC address—Host A's IP address and MAC address.
{
Target IP address—Host B's IP address.
{
Target MAC address—An all-zero MAC address.
All hosts on this subnet can receive the broadcast request, but only the requested host (Host B)
processes the request.
3. Host B compares its own IP address with the target IP address in the ARP request. If they are the
same, Host B:
a. Adds the sender IP address and sender MAC address into its ARP table.
b. Encapsulates its MAC address into an ARP reply.
c. Unicasts the ARP reply to Host A.
4. After receiving the ARP reply, Host A:
a. Adds the MAC address of Host B into its ARP table.
b. Encapsulates the MAC address into the packet and sends the packet to Host B.
Figure 2 ARP address resolution process
If Host A and Host B are on different subnets, Host A sends a packet to Host B as follows:
1. Host A broadcasts an ARP request where the target IP address is the IP address of the gateway.
2. The gateway responds with its MAC address in an ARP reply to Host A.
3. Host A uses the gateway's MAC address to encapsulate the packet, and then sends the packet to
the gateway.
4. If the gateway has an ARP entry for Host B, it forwards the packet to Host B directly. If not, the
gateway broadcasts an ARP request, in which the target IP address is the IP address of Host B.
5. After the gateway gets the MAC address of Host B, it sends the packet to Host B.
ARP table
An ARP table stores dynamic and static ARP entries.
Dynamic ARP entry
ARP automatically creates and updates dynamic entries. A dynamic ARP entry is removed when its aging
timer expires or the output interface goes down. In addition, a dynamic ARP entry can be overwritten by
a static ARP entry.