3Com 4200G 48-Port - Introduction to Gratuitous ARP; Configuring ARP

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mode, all hosts on this subnet can receive the request, but only the requested host (namely, Host B)
will process the request.
3) 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 into its ARP mapping
table, encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.
4) After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP mapping table
for subsequent packet forwarding. Meanwhile, Host A encapsulates the IP packet and sends it out.
Usually ARP dynamically implements and automatically seeks mappings from IP addresses to MAC
addresses, without manual intervention.
Introduction to Gratuitous ARP
The following are the characteristics of gratuitous ARP packets:
z Both source and destination IP addresses carried in a gratuitous ARP packet are the local
addresses, and the source MAC address carried in it is the local MAC addresses.
z If a device finds that the IP addresses carried in a received gratuitous packet conflict with those of
its own, it returns an ARP response to the sending device to notify of the IP address conflict.
By sending gratuitous ARP packets, a network device can:
z Determine whether or not IP address conflicts exist between it and other network devices.
z Trigger other network devices to update its hardware address stored in their caches.
With the gratuitous ARP packet learning function enabled:
A device receiving a gratuitous ARP packet adds the information carried in the packet to its own
dynamic ARP table if it finds no corresponding ARP entry for the ARP packet exists in the cache.
Configuring ARP
Follow these steps to configure ARP basic functions:
To do… Use the command… Remarks
Enter system view
system-view
—
Add a static ARP entry
arp static ip-address
mac-address [ vlan-id
interface-type
interface-number ]
Optional
By default, the ARP mapping
table is empty, and entries are
created dynamically by ARP.
Configure the ARP aging timer
arp timer aging aging-time
Optional
20 minutes by default.
Enable the ARP entry checking
function (that is, disable the
switch from learning ARP
entries with multicast MAC
addresses)
arp check enable
Optional
Enabled by default.