ZyXEL Communications XS3800-28 User Manual page 519

ARP- Re ply
The Switch in ARP-Reply learning mode updates the ARP table only with the ARP replies to the ARP
requests sent by the Switch. This can help prevent ARP spoofing.
In the following example, the Switch does not have IP address and MAC address mapping information
for hosts and in its ARP table, and host
A B
Switch and then sends an ICMP request after getting the ARP reply from the Switch. The Switch finds no
matched entry for host
B
When the Switch receives the ARP reply from host
ICMP request to host . After the Switch gets the ICMP reply from host
B
get host 's MAC address and updates the ARP table with host
A
forward host 's ICMP reply to host
B
G ra tuito us- 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.
Chapter 57 ARP Setup
wants to ping host
A
in the ARP table and broadcasts the ARP request to all the devices on the LAN.
.
A
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. Host sends an ARP request to the
B A
, it updates its ARP table and also forwards host
B
, it sends out an ARP request to
B
's ARP reply. The Switch then can
A
's
A