Arp Implementation - H3C S7500 Series Operation Manual

Operation Manual – ARP
H3C S7500 Series Ethernet Switches
Different manufactures' products may provide more information about ARP table. On
an S7500 series Ethernet switch, you can use the display arp command to display
ARP entries in the table (see ARP Command Manual for details).
Table 1-3
Table 1-3 Description on the fields of an ARP table
IF index
Physical address
IP address
Type

1.1.4 ARP Implementation

The ARP table of a host is empty when the host just starts up. When a dynamic ARP
entry has not been used for a specific time period, it is removed from the ARP table.
The purpose of this is to save memory space and update the entries in the ARP table.
The following describes the implementation procedure of ARP.
Suppose Host A and Host B are on the same network segment. The IP address of
Host A is IP_A and that of Host B is IP_B. To send a packet to Host B, Host A first
looks up its own ARP table for an ARP entry that contains IP_B. If such an entry is
found, Host A encapsulates the IP packet into a frame by using the MAC address
in the entry as the destination MAC address and then sends the frame to Host B.
If no such an entry is found in the ARP table, Host A puts the packet into the
transmission queue, generates an ARP request packet with the IP addresses of
Host B and Host A and the MAC address of Host A (IP_B, IP_A and MAC_A), and
broadcasts the request on the Ethernet. Since the ARP request is broadcasted, all
hosts on the network segment receive it. However, only the host with IP address
IP_B (Host B) will process the request further.
Host B adds the sender IP address and MAC address carried in the request (IP_A
and MAC_A of Host A) in an entry to its ARP table and then returns an ARP reply
packet to the sender (Host A), with its MAC address carried in the packet. Note
that the ARP reply is a unicast packet instead of a broadcasted packet.
Upon receiving the ARP reply, Host A extracts the IP address and MAC address of
Host B from the packet, adds them in an entry to its ARP table, and then sends out
all the packets destined for Host B in the transmission queue.
describes the fields of APR table.
Field
Index of the physical interface or port on the device owning
the physical address and IP address in the entry
Physical address of the device, a MAC address
IP address of the device
Entry type, which can be:
1: Not any of the following
2: Invalid entry
3: Dynamic entry
4: Static entry
Description
1-3
Chapter 1 ARP Configuration