3Com 3CR17660-91, Configuration Manual

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198 C
HAPTER
25: ARP C
ONFIGURATION
By sending gratuitous ARP packets, a network device can:
■
Determine whether or not IP address conflicts exist between it and other network
devices.
■
Trigger other network devices to update its hardware address stored in their
caches.
When the gratuitous ARP packet learning function is enabled on a switch and the
switch receives a gratuitous ARP packet, the switch updates the existing ARP entry
(contained in the cache of the switch) that matches the received gratuitous ARP
packet using the hardware address of the sender carried in the gratuitous ARP packet.
A switch operates like this whenever it receives a gratuitous ARP packet.
ARP Configuration
ARP entries in an S4200G series Ethernet switch are classified into static entries and
dynamic entries, as described in Table 166.
Adding a Static ARP
Mapping Entry Manually
CAUTION:
Static ARP mapping entries are valid as long as the Ethernet switch operates. But
operations that invalidate ARP entries, such as changing/removing VLAN interfaces,
removing VLANs, or removing ports from VLANs, may cause the corresponding ARP
entries being removed automatically.
As for the arp static command, the value of the vlan-id argument must be the ID of
an existing VLAN, and the port identified by the interface-type and interface-number
arguments must belong to the VLAN.
Configuring the ARP
Aging Timer for
Dynamic ARP Entries
The ARP aging timer applies to all dynamic ARP mapping entries.
Table 166 ARP entries
ARP entry Generation Method Maintenance Mode
Static ARP entry Manually configured Manual maintenance
Dynamic ARP entry Dynamically generated ARP entries of this type age with
time. The aging period is set by the
ARP aging timer.
Table 167 Add a static ARP mapping entry manually
Operation Command Description
Enter system view system-view
Add a static ARP
mapping entry
manually arp static ip-address mac-address
[ vlan-id interface-type
interface-number ]
Required
The ARP mapping table is empty
when a switch is just started. And
the address mapping entries are
created by ARP.
Table 168 Configure the ARP aging timer for dynamic ARP entries
Operation Command Description
Enter system view system-view
Configure the ARP
aging timer arp timer aging aging-time Optional
By default, the ARP aging timer is
set to 20 minutes.