Arp Configuration; Configure Static Arp - 3Com 7700 Configuration Manual

50 C 4: N
HAPTER ETWORK

ARP Configuration

Example: IP Address
Resolution

Configure Static ARP

P O
ROTOCOL PERATION
but not receive the ARP packets, there are probably errors on the Ethernet physical
layer.
An IP address cannot be directly used for communication between network
devices because devices can only identify MAC addresses. An IP address is only the
address of a host in the network layer. To send data packets through the network
layer to the destination host, the physical address of the host is required. So the IP
address must be resolved to a physical address.
When two hosts in Ethernet communicate, they must know the MAC addresses of
each other. Every host maintains the IP-MAC address translation table, which is
known as the ARP mapping table. A series of maps between IP addresses and
MAC addresses of other hosts are stored in the ARP mapping table. When an ARP
mapping entry is not in use for a long time, the host will remove it from the
mapping table to save memory space and shorten the search interval.
Suppose there are two hosts on the Ethernet: 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. Since the ARP request packet is broadcast, all hosts on the
Ethernet receive the request. However, only the requested host (i.e., 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 and add MAC address of
Host B to send it to Host A. The reply packet will be sent directly to Host A instead
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 executes and automatically 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. Addresses
that are mapped manually are referred to as static ARP. The user can display, add,
or delete the entries in the ARP mapping table through manual commands.
The static ARP configuration includes:
Manually Add/delete static ARP Mapping Entries
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Display and debug ARP
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