ARP Commands 649Command ModeGlobal Configuration modeUser GuidelinesWhen an ARP entry reaches its maximum age, the system must decidewhether to retain or delete the entry. If the entry has recently been used toforward data packets, the system will renew the entry by sending an ARPrequest to the neighbor. If the neighbor responds, the age of the ARP cacheentry is reset to 0 without removing the entry from the hardware. Traffic tothe host continues to be forwarded in hardware without interruption. If theentry is not being used to forward data packets, then the entry is deleted fromthe ARP cache, unless the dynamic renew option is enabled. If the dynamicrenew option is enabled, the system sends an ARP request to renew the entry.When an entry is not renewed, it is removed from the hardware andsubsequent data packets to the host trigger an ARP request. Traffic to thehost is lost until the router receives an ARP reply from the host. Gatewayentries, entries for a neighbor router, are always renewed. The dynamic renewoption only applies to host entries.The disadvantage of enabling dynamic renew is that once an ARP cache entryis created, that cache entry continues to take space in the ARP cache as longas the neighbor continues to respond to ARP requests, even if no traffic isbeing forwarded to the neighbor. In a network where the number of potentialneighbors is greater than the ARP cache capacity, enabling dynamic renewcould prevent some neighbors from communicating because the ARP cache isfull.Exampleconsole#configureconsole(config)#arp dynamicrenewconsole(config)#no arp dynamicrenewarp purgeUse the arp purge command in Privileged EXEC mode to cause the specifiedIP address to be removed from the ARP cache. Only entries of type dynamicor gateway are affected by this command.
