3Com 2928 User Guide - Page 341

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: z 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. z 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. z 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. z 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. ARP Table After obtaining the MAC address for the destination host, the device puts the IP-to-MAC mapping into its own ARP table. This mapping is used for forwarding packets with the same destination in future. An ARP table contains ARP entries, which fall into one of two categories: dynamic or static. Dynamic ARP entry A dynamic entry is automatically created and maintained by ARP. It can get aged, be updated by a new ARP packet, or be overwritten by a static ARP entry. When the aging timer expires or the interface goes down, the corresponding dynamic ARP entry will be removed. 1-2