ARP Overview 551Figure 156 ARP address resolution processWhen Host A and Host B are not on the same subnet, Host A first sends an ARPrequest to the gateway. The destination IP address in the ARP request is the IPaddress of the gateway. After obtaining the MAC address of the gateway from anARP reply, Host A encapsulates the packet and sends it to the gateway.Subsequently, the gateway broadcasts the ARP request, in which the destination IPaddress is the one of Host B. After obtaining the MAC address of Host B fromanother ARP reply, the gateway sends the packet to Host B.ARP Mapping Table After obtaining the destination MAC address, the device adds the IP-to-MACmapping into its own ARP mapping table. This mapping is used for forwardingpackets with the same destination in future.An ARP mapping table contains ARP entries, which fall into two categories:dynamic and static.1 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. Whenthe aging timer expires or the interface goes down, the corresponding dynamicARP entry will be removed.2 A static ARP entry is manually configured and maintained. It cannot get aged or beoverwritten by a dynamic ARP entry. It can be permanent or non-permanent.■ A permanent static ARP entry can be directly used to forward data. Whenconfiguring a permanent static ARP entry, you must configure a VLAN andoutbound interface for the entry besides the IP address and MAC address.■ A non-permanent static ARP entry cannot be directly used for forwarding data.When configuring a non-permanent static ARP entry, you only need toconfigure the IP address and MAC address. When forwarding IP packets, thedevice sends an ARP request. If the source IP and MAC addresses in thereceived ARP reply are the same as the configured IP and MAC addresses, thedevice adds the interface receiving the ARP reply into the static ARP entry. Nowthe entry can be used for forwarding IP packets.n Usually ARP dynamically implements and automatically seeks mappings from IPaddresses to MAC addresses, without manual intervention.Target IPaddress192 .168 .1 .1Target IPaddress192 .168 .1 .2Host A192.168.1.10002- 6779-0 f4cHost B192.168.1.200a 0-2470 -febdTarget MACaddress0000 -0000- 0000Sender MACaddress00a 0-2470 -febdTarget MACaddress0002 -6779 -0f4cSender IPaddress192.168 .1.1Sender MACaddress0002 -6779 -0f4cSender IPaddress192.168 .1.2
