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Chapter 33 ARP Learning

GS2200-8/24 User’s Guide

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ARP request to the Switch and then sends an ICMP request after getting the ARP reply from the

Switch. The Switch finds no matched entry for host B in the ARP table and broadcasts the ARP

request to all the devices on the LAN. When the Switch receives the ARP reply from host B, it

updates its ARP table and also forwards host A’s ICMP request to host B. After the Switch gets the

ICMP reply from host B, it sends out an ARP request to get host A’s MAC address and updates the

ARP table with host A’s ARP reply. The Switch then can forward host B’s ICMP reply to host A.

Gratuitous-ARP

A gratuitous ARP is an ARP request in which both the source and destination IP address fields are

set to the IP address of the device that sends this request and the destination MAC address field is

set to the broadcast address. There will be no reply to a gratuitous ARP request.

A device may send a gratuitous ARP packet to detect IP collisions. If a device restarts or its MAC

address is changed, it can also use gratuitous ARP to inform other devices in the same network to

update their ARP table with the new mapping information.

In Gratuitous-ARP learning mode, the Switch updates its ARP table with either an ARP reply or a

gratuitous ARP request.

ARP-Request

When the Switch is in ARP-Request learning mode, it updates the ARP table with both ARP replies,