Configuring Arp - 3Com Switch 4500 26-Port Configuration Manual

84 C 5: N
HAPTER ETWORK

Configuring ARP

P O
ROTOCOL PERATION
dynamic ARP mapping entry is not in use for a specified period of time, the host
will remove it from the ARP mapping table so as to save the memory space and
shorten the interval for Switch to search ARP mapping table.
Suppose there are two hosts on the same network segment: Host A and Host B.
The IP address of Host A is IP_A and the IP address of Host B is IP_B. Host A will
transmit messages to Host B. Host A checks its own ARP mapping table first to
make sure whether there are corresponding ARP entries of IP_B in the table. If the
corresponding MAC address is detected, Host A will use the MAC address in the
ARP mapping table to encapsulate the IP packet in frame and send it to Host B. If
the corresponding MAC address is not detected, Host A will store the IP packet in
the queue waiting for transmission, and broadcast it throughout the Ethernet. The
ARP request packet contains the IP address of Host B and IP address and MAC
address of Host A. Because the ARP request packet is broadcast, all hosts on the
network segment can receive the request. However, only the requested host (that
is, Host B) needs to process the request. Host B will first store the IP address and
the MAC address of the request sender (Host A) in the ARP request packet in its
own ARP mapping table. Then Host B will generate an ARP reply packet, into
which it will add MAC address of Host B, and then send it to Host A. The reply
packet will be directly sent to Host A in stead of being broadcast. Receiving the
reply packet, Host A will extract the IP address and the corresponding MAC
address of Host B and add them to its own ARP mapping table. Then Host A will
send Host B all the packets standing in the queue.
Normally, dynamic ARP automatically executes and searches for the resolution
from the IP address to the Ethernet MAC address without the administrator.
The ARP mapping table can be maintained dynamically or manually. Usually, the
manually configured mapping from the IP addresses to the MAC addresses is
known as static ARP. The user can display, add, or delete entries in the ARP
mapping table through relevant manual maintenance commands.
Static ARP configuration is described in the following sections:
Manually Adding/Deleting Static ARP Mapping Entries
■
Configuring the Dynamic ARP Aging Timer
■
Configuring the Creation of ARP Entries for Multicast Packets
■
Manually Adding/Deleting Static ARP Mapping Entries
You can configure static ARP mapping items either in System View or Ethernet
Port View. In System View, you can configure global static ARP mapping entries,
or configure static ARP mapping entries for the designated egress port; while in
Ethernet Port View, you may set the current port as the egress port of static ARP.
Perform the following configuration in System View or Ethernet Port View.
Table 79 Manually Adding/Deleting Static ARP Mapping Entries
Operation
Manually add a static ARP
mapping entry (System View)
Command
arp static ip_address mac_address [ vlan_id
{ interface_type interface_num |
interface_name } ]