Automatic Renewal

Lease renewal is something that is continually happening on all DHCP clients for which the lease duration has not been set to unlimited. By definition, lease renewal is the process by which a configured DHCP client tries to renegotiate its current IP information and options with its leasing DHCP server. Lease renewal is an automatic process and is determined by the lease duration settings in the properties of the DHCP scope to which the client's address belongs.

The beginning of the lease renewal process happens when 50 percent of the client's lease duration interval has elapsed. For example, if a computer named CLIENTX has a lease duration of eight days, it will try to renew its lease after four days. The renewal begins at the third stage of the DHCP lease process; issuing a DHCPREQUEST message to the same server that originally leased the IP address to the client (remember, this is not a broadcast message as were all the messages in the original DORA process). If the DHCP server is available, the client receives a DHCPACK from the server, renewing the client lease in accordance with the lease duration interval, and updates any DHCP options that were changed.

If the originating DHCP server is unavailable at the 50 percent mark, the client waits until 87.5 percent of the lease is up and tries again with another DHCPREQUEST message.

If at this time the DHCP server is still unavailable, the client will keep its current IP address only until the lease expires, at which time it will begin at stage one of the DHCP lease process, broadcasting a DHCPDISCOVER message in an effort to find a new DHCP server.

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