DHCP servers raise level of fault tolerance

In most cases, you'll want to configure at least two DHCP servers on the network. If they are configured properly, having multiple DHCP servers increases reliability and allows for fault tolerance.

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As part of planning, you must consider how many DHCP servers should be made available on the network. In most cases, you'll want to configure at least two DHCP servers. If they are configured properly, having multiple DHCP servers increases reliability and allows for fault tolerance.

In a large enterprise, a server cluster can be your primary mechanism for ensuring DHCP availability and providing for fault tolerance. Here, if a DHCP server fails, the DHCP Server service can be failed over to another server in the cluster, allowing for seamless transition of DHCP services.

Although you can configure the DHCP Server service for failover on a cluster, much simpler and less expensive fault-tolerance implementations are available, and these implementations work with any size network. The implementations include 50/50 failover, 80/20 failover and 100/100 failover options.

50/50 failover

By configuring the 50/50 failover approach, you use two DHCP servers to make an equal amount of IP addresses available to clients for leasing. Here, each DHCP server is configured with an identical scope range but with different exclusions within that range. The first server gets the first half of the scope's IP address range and excludes the second half. The second server gets the second half of the scope's IP address range and excludes the first half.

Although this approach is designed to provide some redundancy and fault tolerance, it is possible that one of the servers would assign more IP addresses than the other. This could lead to a situation in which one of the servers doesn't have any available IP addresses, and if it is the other server that fails, no IP addresses would be available to clients seeking new leases and they would be configured to use the APIPA feature of Windows.

80/20 failover

By configuring the 80/20 failover approach, you use two DHCP servers to make a disproportionate amount of IP addresses available to clients for leasing. Here, you have a primary DHCP server that is configured with 80 percent of the available IP addresses and a backup DHCP server that is configured with 20 percent of the available IP addresses. This situation is ideal when the DHCP servers are separated from each other, such as when the primary DHCP server is on the primary subnet and the backup DHCP server is on a smaller remote subnet.

Although this approach is designed to provide some redundancy and fault tolerance, it is possible that the primary would be offline too long and the backup DHCP server would run out of available IP addresses. If this were to happen, no IP addresses would be available to clients seeking new leases, and they would be configured to use APIPA.

100/100 failover

By configuring the 100/100 failover approach, you make twice as many IP addresses available as are needed. Thus, if you must provide DHCP services for 200 clients, you make at least 400 IP addresses available to those clients. As with 50/50 failover, each DHCP server is configured with an identical scope range but with different exclusions within that range. The first server gets the first half of the scope's IP address range and excludes the second half. The second server gets the second half of the scope's IP address range and excludes the first half.

To make twice as many IP addresses available as are needed, you must think carefully about the IP address class you use and would most likely want to use a Class A or Class B network. With this in mind, the organization's two DHCP servers might be configured as follows:

Because more than two times as many IP addresses are available, every client on the network can obtain a lease even if one of the DHCP servers goes offline. Not only does this approach offer availability and fault tolerance, it gives you flexibility. You are able to take one of the DHCP server's offline and perform maintenance or upgrades without worrying about running out of available IP addresses.

About the author: Rahul Shah currently works at a software firm in India, where he is a systems administrator maintaining Windows servers. He has also worked for various software firms in testing and analytics, and also has experiences deploying client/server applications in different Windows configurations.

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This was first published in July 2006

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