ZyXEL Communications GS1920v2 Series User Manual page 324

Chapter 52 ARP Setup
In the following example, the Switch does not have IP address and MAC address mapping information
for hosts A and B in its ARP table, and host A wants to ping host B. Host A sends an 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.
Therefore in the following example, the Switch can learn host A's MAC address from the ARP request
sent by host A. The Switch then forwards host B's ICMP reply to host A right after getting host B's MAC
GS1920v2 Series User's Guide
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