DHCP Renewal Interval

In class we had a discussion on a slide that seems to present the time when a DHCP client at 87.5% of the lease period instead of 50%. Below is an excerpt for TechNet that should help clear up the confusion.

Renewing a lease

The DHCP client first attempts to renew its lease when 50 percent of the original lease time, known as T1, has passed. At this point, the DHCP client sends a unicast DHCPRequest message to the DHCP server that originally granted its lease. If the server is available, and the lease is still available, the server responds with a unicast DHCPAck message, and the lease is renewed.

If the original DHCP server is available, but the client’s current lease is no longer available, the DHCP server responds with a DHCPNack message, and the client immediately starts the process to obtain a new lease. This can happen if the client has changed subnets or if the DHCP server cannot fulfill the lease request for some other reason.

If there is no response from the DHCP server, the client waits until 87.5 percent of the lease time, known as T2, has passed. At T2, the client enters the rebinding state, and broadcasts a DHCPRequest message to attempt to renew the lease from any available DHCP server. If no DHCP server is available by the time the lease expires, the client immediately unbinds itself from the existing lease and starts the process to obtain a new lease, beginning with a DHCPDiscover message.

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