Chapter 1 Arp Configuration; Introduction To Arp - Huawei Quidway S3000-EI Series Operation Manual

Operation Manual - Network Protocol
Quidway S3000-EI Series Ethernet Switches

Chapter 1 ARP Configuration

1.1 Introduction to ARP

I. Necessity of ARP
An IP address cannot be directly used for communication between network devices
because network devices can only identify MAC addresses. An IP address is only an
address of a host in the network layer. To send the data packets transmitted through the
network layer to the destination host, physical address of the host is required. So the IP
address must be resolved into a physical address.
II. ARP implementation procedure
When two hosts on the Ethernet communicate, they must know the MAC addresses of
each other. Every host will maintain the IP-MAC address translation table, which is
known as ARP mapping table. A series of maps between IP addresses and MAC
addresses of other hosts which were recently used to communicate with the local host
are stored in the ARP mapping table. When a 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. Since
the ARP request packet is broadcast, all hosts on the network segment can 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 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.
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Chapter 1 ARP Configuration