HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Services Conf - Page 10

ARP table, Source IP address and source MAC address, Target IP address

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1. Host A looks in 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 using the following information: { Source IP address and source MAC address-Host A's own IP address and the 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 to its ARP table. b. Encapsulates the MAC address into the packet and sends it to Host B. Figure 2 ARP address resolution process If Host A and Host B are on different subnets, the resolution process is as follows: 1. Host A sends an ARP request to the gateway. The target IP address in the ARP request is the IP address of the gateway. 2. After obtaining the MAC address of the gateway from an ARP reply, Host A sends the packet to the gateway. 3. 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. 4. After obtaining the MAC address of Host B, the gateway sends the packet to Host B. ARP table An ARP table stores dynamic and static ARP entries. 2

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2
1.
Host A looks in 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 using
the following information:
{
Source IP address and source MAC address
—Host A’s own IP address and the 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 to its ARP table.
b.
Encapsulates the MAC address into the packet and sends it to Host B.
Figure 2
ARP address resolution process
If Host A and Host B are on different subnets, the resolution process is as follows:
1.
Host A sends an ARP request to the gateway. The target IP address in the ARP request is the IP
address of the gateway.
2.
After obtaining the MAC address of the gateway from an ARP reply, Host A sends the packet to the
gateway.
3.
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.
4.
After obtaining the MAC address of Host B, the gateway sends the packet to Host B.
ARP table
An ARP table stores dynamic and static ARP entries.