Arp Configuration; Displaying And Debugging Arp - 3Com 4210 PWR Configuration Manual

ARP Process

ARP Configuration

Displaying and
Debugging ARP
Suppose that Host A and Host B are on the same subnet and that Host A sends a
message 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 for
Host B. If Host A finds it, 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.
2 If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an
ARP request, in which the source IP address and source MAC address are
respectively the IP address and MAC address of Host A and the destination IP
address and MAC address are respectively the IP address of Host B and an all-zero
MAC address. Because the ARP request is sent in broadcast 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.
CAUTION:
Static ARP entries are valid as long as the Ethernet switch operates normally.
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But some operations, such as removing a VLAN, or removing a port from a
VLAN, will make the corresponding ARP entries invalid and therefore removed
automatically.
As for the arp static command, the value of the vlan-id argument must be the
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ID of an existing VLAN, and the port identified by the interface-type and
interface-number arguments must belong to the VLAN.
Currently, static ARP entries cannot be configured on the ports of an
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aggregation group.
After the above configuration, you can execute the display command in any view
to display the running of the ARP configuration, and to verify the effect of the
configuration.
You can execute the reset command in user view to clear ARP entries.
ARP Configuration 279