ZyXEL P-662HW-63 User Guide - Page 440
ARP Behavior and the ARP ackGratuitous Commands
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Appendix J Command Interpreter Figure 299 Routing Command Example ras> ip nat routing 2 0 Routing can work in NAT when no NAT rule match LAN: yes ARP Behavior and the ARP ackGratuitous Commands The ZyXEL Device does not accept ARP reply information if the ZyXEL Device did not send out a corresponding request. This helps prevent the ZyXEL Device from updating its ARP table with an incorrect IP address to MAC address mapping due to a spoofed ARP. An incorrect IP to MAC address mapping in the ZyXEL Device's ARP table could cause the ZyXEL Device to send packets to the wrong device. Commands for Using or Ignoring Gratuitous ARP Requests A host can send an ARP request to resolve its own IP address. This is called a gratuitous ARP request. The packet uses the host's own IP address as the source and destination IP address. The packet uses the Ethernet broadcast address (FF:FF:FF:FF:FF:FF) as the destination MAC address. This is used to determine if any other hosts on the network are using the same IP address as the sending host. The other hosts in the network can also update their ARP table IP address to MAC address mappings with this host's MAC address. The ip arp ackGratuitous commands set how the ZyXEL Device handles gratuitous ARP requests. • Use ip arp ackGratuitous active no to have the ZyXEL Device ignore gratuitous ARP requests. • Use ip arp ackGratuitous active yes to have the ZyXEL Device respond to gratuitous ARP requests. For example, say the regular gateway goes down and a backup gateway sends a gratuitous ARP request. If the request is for an IP address that is not already in the ZyXEL Device's ARP table, the ZyXEL Device sends an ARP request to ask which host is using the IP address. After the ZyXEL Device receives a reply from the backup gateway, it adds an ARP table entry. If the ZyXEL Device's ARP table already has an entry for the IP address, the ZyXEL Device's response depends on how you configure the ip arp ackGratuitous forceUpdate command. • Use ip arp ackGratuitous forceUpdate on to have the ZyXEL Device update the MAC address in the ARP entry. • Use ip arp ackGratuitous forceUpdate off to have the ZyXEL Device not update the MAC address in the ARP entry. A backup gateway (as in the following graphic) is an example of when you might want to turn on the forced update for gratuitous ARP requests. One day gateway A shuts down and the backup gateway (B) comes online using the same static IP address as gateway A. Gateway B broadcasts a gratuitous ARP request to ask which host is using its IP address. If ackGratuitous is on and set to force updates, the ZyXEL Device receives the gratuitous ARP request and 440 P-662H/HW-D Series User's Guide