ZyXEL Communications GS2200 Series, User Manual

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Chapter 33 ARP Learning
GS2200 Series 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,
gratuitous ARP requests and ARP requests.
A
B
ARP Request
ARP Reply
ICMP Request
ARP Request
ARP Reply
ICMP Request
ICMP Reply
ARP Request
ARP Reply
ICMP Reply