ZyXEL XGS4700-48F User Guide - Page 380

Chapter 42 ARP Learning 42.1.2.1 ARP-Reply By default, the Switch is in ARP-Reply learning mode and 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 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. A ARP Request ARP Reply ICMP Request ARP Request ARP Reply ICMP Reply B ARP Request ARP Reply ICMP Request ICMP Reply 42.1.2.2 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 380 XGS4700-48F User's Guide