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DHCP Availability

Updated: November 10, 2008

Applies To: Windows Server 2008

Availability of the IP addressing schema is determined by the method of allocation and your organizational needs. If you are using a manually configured method, you might consider using a spreadsheet on a server so that other people can access the file. You can then replicate that file to other servers to ensure its availability even if the primary server is unavailable.

If you choose an automated allocation scheme using DHCP, you will need to consider methods to ensure that your DHCP service will always be able to respond to DHCP/BOOTP requests. This can be accomplished through split scopes, a standby server, failover to a network device that supports DHCP/BOOTP allocation, or through the use of a server cluster. For more information about these design options, see Design Step 1: Determine the DHCP Service Design.

If clients receive address allocations from a DHCP server, it is important to be able to predict how they will be affected by any DHCP server downtime. In general, the longer the lease period, the lesser the effects will be if the DHCP downtime remains short. For example, if client lease periods are set to the default of 8 days, clients do not attempt to renew the lease until 50 percent of this period (4 days) has lapsed. If the original DHCP server is unavailable at this time, the client continues with this leased address until 87.5 percent of the lease period (7 days) and then attempts to renew with any DHCP server. With clients attempting to renew after 4 days, even if the DHCP server were to remain unavailable for 2 days, clients would not reach the 87.5 percent rebinding state. Therefore, you do not normally need to worry about any outage that is within 25 percent of the lease duration. Similarly, the shorter the lease times, the shorter the time available to recover the DHCP server.

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