Design Option 3: Standby Servers
Updated: November 10, 2008
Applies To: Windows Server 2008
A hot standby DHCP server is another server that is installed and configured identically to the primary DHCP server; the only difference is that the standby server and its scopes are not activated for use under normal conditions. The duplicate scopes are activated only when a primary DHCP server has stopped or needs to be taken offline for an extended period of time. DHCP in Windows 2000 Server, Windows Server 2003, and Windows Server 2008 can be configured to detect IP conflicts before providing the address to a client; this feature prevents the standby server from leasing IP addresses that have already been leased to clients from the original DHCP server. This feature should not be run all the time due to the network traffic it generates; it is best used to selectively handle outages.
The advantages of standby servers include:
Economical solution: This is a cost-effective solution because the extra expense and administration of clustering is not required. The standby server can be configured on an existing server, such as a WINS or DNS server.
100 percent scope availability: This option enables the environment to continue operating without the overhead costs of managing split scopes.
The disadvantages of standby servers include:
Requires manual intervention: The hot standby solution requires careful configuration and manual administration to ensure failover transition. Therefore, it is less recommended as a planning alternative than the use of two to three DHCP servers that balance active scope use.
DNS record ownership: This solution can cause problems if DHCP is configured to update DNS records because DNS records owned by the failed DHCP server cannot be updated by the standby replacement. For more information, see DHCP Interoperability.
Extra network bandwidth utilization: There can be performance problems with this solution, at least when the standby server is initially brought online. Because the standby server will not have knowledge of the addresses that have already been leased, it will attempt to assign addresses already assigned to clients. If conflict detection is turned on as recommended for standby servers, the DHCP server pings each address to check whether it is in use before assigning it. When a standby server is brought online, it is likely that it will need to ping a large number of addresses before finding one that is not already in use.