ARP Configuration 279ARP Process Suppose that Host A and Host B are on the same subnet and that Host A sends amessage to Host B. The resolution process is as follows:1 Host A looks in its ARP mapping table to see whether there is an ARP entry forHost B. If Host A finds it, Host A uses the MAC address in the entry to encapsulatethe 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 anARP request, in which the source IP address and source MAC address arerespectively the IP address and MAC address of Host A and the destination IPaddress and MAC address are respectively the IP address of Host B and an all-zeroMAC address. Because the ARP request is sent in broadcast mode, all hosts on thissubnet can receive the request, but only the requested host (namely, Host B) willprocess the request.3 Host B compares its own IP address with the destination IP address in the ARPrequest. If they are the same, Host B saves the source IP address and source MACaddress into its ARP mapping table, encapsulates its MAC address into an ARPreply, and unicasts the reply to Host A.4 After receiving the ARP reply, Host A adds the MAC address of Host B into its ARPmapping table for subsequent packet forwarding. Meanwhile, Host Aencapsulates the IP packet and sends it out.Usually ARP dynamically implements and automatically seeks mappings from IPaddresses to MAC addresses, without manual intervention.ARP Configuration CAUTION:■ Static ARP entries are valid as long as the Ethernet switch operates normally.But some operations, such as removing a VLAN, or removing a port from aVLAN, will make the corresponding ARP entries invalid and therefore removedautomatically.■ As for the arp static command, the value of the vlan-id argument must be theID of an existing VLAN, and the port identified by the interface-type andinterface-number arguments must belong to the VLAN.■ Currently, static ARP entries cannot be configured on the ports of anaggregation group.Displaying andDebugging ARPAfter the above configuration, you can execute the display command in any viewto display the running of the ARP configuration, and to verify the effect of theconfiguration.You can execute the reset command in user view to clear ARP entries.
