Configuring Arp - 3Com 5500-SI Configuration Manual

274 C 17: N
HAPTER ETWORK

Configuring ARP

P O
ROTOCOL PERATION
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 269 Manually Adding/Deleting Static ARP Mapping Entries
Operation
Manually add a static ARP mapping
entry (System View)
Manually add a static ARP mapping
entry (Ethernet Port View)
Manually delete a static ARP mapping
entry (System View or Ethernet Port
View)
By default, the ARP mapping table is empty and the address mapping is obtained
through dynamic ARP.
Command
arp static ip_address mac_address [ vlan_id
{ interface_type interface_num |
interface_name } ]
arp static ip_address mac_address vlan_id
undo arp ip_address