ZyXEL Communications P-791R, User Manual

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Appendix G Command Interpreter
P-791R v2 User’s Guide
328
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
updates its ARP table. This way the ZyXEL Device has a correct gateway ARP entry to
forward packets through the backup gateway. If ackGratuitous is off or not set to force
updates, the ZyXEL Device will not update the gateway ARP entry and cannot forward
packets through gateway B.