Windows Server 2008 : DHCP/WINS/Domain Controllers - Implementing Redundant DHCP Services

2/18/2011 2:39:39 PM
DHCP administrators who recognize the need to provide redundancy for DHCP have been challenged for many years and have had to implement manual configurations to provide any level of redundancy. Many of these implementations lacked certain functionality and required network resources that were not always readily available, such as a suitable second server to deploy DHCP services on. DHCP services redundancy can be achieved by either deploying multiple DHCP servers running overlapping or split scopes or by deploying clustered DHCP services. Many organizations do not have the administrative support or budget to deploy clustered DHCP services, so the more common approach to providing DHCP redundancy is to deploy multiple DHCP servers running split scopes.

DHCP Split Scope

A DHCP scope is primarily defined by an address pool that contains the IP addresses that will be made available to DHCP clients. Within a scope, there is usually an included and excluded IP address list as well as DHCP scope options, such as default gateway and DNS server options, which will be delivered to clients receiving a DHCP IP address lease. A scope also contains IP address reservations and other general scope properties that enable administrators to define how the DHCP server will deal with Dynamic DNS registration for DHCP leases, audit log path settings, Name Protection settings, and much more. When redundancy is required for DHCP services, and deploying DHCP services on a cluster is not a viable option, DHCP administrators will deploy multiple DHCP servers set up in a split-scope configuration.

A DHCP split scope is a range of IP addresses available for DHCP IP address leases that are logically split between two or more DHCP servers. The IP address pool is the same on both servers, and the defining configuration for a split scope is the excluded IP range. For example, suppose a DHCP administrator was given an address pool of to On a split-scope configuration, both DHCP servers would have this range defined in the scope, but on the first DHCP server, there would be an excluded address range of to; this means that the first DHCP server would lease addresses to The second DHCP server would also have to defined in the included address pool, but the excluded address range would be to With this configuration, the second DHCP server would lease addresses from to With a split-scope configuration, if a single DHCP server becomes unavailable, the secondary DHCP server can still provide DHCP leases on the network to which the split scope applies.

Historically, a split-scope configuration needed to be manually created by DHCP administrators, but starting with Windows Server 2008 R2, Microsoft now includes a DHCP Split-Scope Configuration Wizard. This wizard allows a DHCP administrator to take an existing scope on the primary DHCP server and run the wizard to duplicate the scope on a designated secondary DHCP server and define how the addresses will be split among the two servers. This wizard will make the necessary changes to both of the DHCP servers, leaving less room for user error. But before the DHCP Split-Scope Configuration Wizard can be run, a DHCP administrator must consider how the scope will be split, and the following section describes three common split-scope configurations that should be considered. The process of splitting an existing DHCP scope is detailed later in this chapter.

Examining the 50/50 Split-Scope Configuration

The 50/50 split-scope configuration includes two DHCP servers, in which each DHCP server is configured with the same address range for the address pool, but each must have a different excluded IP address and the total number of addresses is split in half or 50/50. Figure 1 illustrates the 50/50 split-scope configuration. As indicated in the diagram, the network has 200 clients defined by Each DHCP server contains a scope to cover the entire specific client subnet. Server1’s scope is configured with exclusions for all IP addresses except for the range of– Server2’s scope is configured with exclusions for the first half and a client lease range of–

Figure 1. Examining the 50/50 failover approach.

Upon requesting a client IP address, the first server to respond to a request will be accepted, thus roughly balancing the load between the two servers, except for one thing: There is no way to determine which DHCP server will respond first and serve the client requests, so there is a chance that one DHCP server will run out of IP addresses before all IP addresses are used. Also, another issue with this configuration is that both DHCP servers would respond to lease requests and a DHCP administrator would need to review both servers to troubleshoot and determine what the true number of available IP addresses are, when clients are having issues getting an IP address lease.

Exploring the 80/20 Failover Approach to DHCP Fault Tolerance

The 80/20 failover approach is similar to the 50/50 approach, except that the effective scope range on the server designated as the backup DHCP server contains only 20% of the available client IP range. The server with 80% of the range would be considered the primary DHCP server, and the 20% server would be considered the secondary. In the event of primary server failure, the secondary server would have enough IP addresses to provide leases until the primary server could be fixed and returned to operation. This is the best-practice split-scope configuration, but until Windows Server 2008 R2, this configuration frequently resulted in the secondary server running out of IP addresses during regular operation because it can respond to client requests as fast as the primary server—and the first server to respond wins!

Understanding the 100/100 Failover Approach to DHCP Fault Tolerance

The 100/100 split-scope configuration in Windows Server 2008 R2 DHCP can be the most effective means of achieving high availability out of a DHCP environment. The 100/100 split-scope configuration, in its simplest form, is the same as the 50/50 except that the total scope range contains at least twice the number of total DHCP clients.

In Figure 2, the subnet has a total of 750 clients. This subnet is serviced by two DHCP servers, each of which has a scope for the subnet. Each server has a scope with addresses from through The scope on Server1 excludes all IP addresses except those in the range of through The scope on Server2 excludes all IP addresses except those in the range from through Each effective range is subsequently large enough to handle 1,000 clients, which is more than enough for every machine on the network.

Figure 2. The 100/100 failover approach.

If one of the DHCP servers experiences an interruption in service, and it no longer responds, the second server will take over, responding to clients and enabling them to change their IP addresses to the IP addresses available in the separate range. With this configuration, extended downtime of a single DHCP server can be tolerated without much loss of functionality.

The main caveat to this approach is that a large number of IP addresses must be available for clients, more than twice the number than would normally be available. This might prove to be difficult, if not impossible, in many networks that have a limited IP range to work with, and is especially true when deploying new DHCP services on existing or established networks. However, in organizations with a larger IP range, such as those offered by private Class A network configurations (10.x.x.x and so on), this type of configuration might be ideal.

As you can see in Figure 11.9, both servers are configured with the same IP address range but even with the exclusion range, each server individually contains enough IP addresses to serve the entire DHCP client base.

Windows Server 2008 R2 Delay Configuration Setting

Starting with Windows Server 2008 R2, the DHCP Server service now includes an IPv4 scope setting named Delay Configuration. The Delay Configuration setting is configured on the Advanced Scope Properties page and allows a DHCP administrator to delay the response from a DHCP server, to ensure that the desired primary DHCP server answers all DHCP lease requests, unless it is out of service. With this new setting alone, DHCP administrators can simplify the management of a split-scope DHCP configuration; as during normal operation, all leases should be only on the primary server. The Delay Configuration setting should be set up on secondary DHCP server scope properties. With this setting, the 80/20 best-practice split scope can be used confidently. To enable the Delay Configuration setting on a secondary DHCP server scope, simply open the scope properties from the DHCP server console, select the Advanced tab, and near the bottom of the window, type in the number of milliseconds the DHCP server should wait before responding to a client lease request, as shown in Figure 3.

Figure 3. Setting the DHCP scope Delay Configuration setting.

DHCP Split-Scope Configuration Wizard

When deploying multiple DHCP servers in a split-scope configuration is desired, it is recommended to use the new DHCP Split-Scope Configuration Wizard. The DHCP Split-Scope Configuration Wizard will create the new scope on the secondary DHCP server and will even copy client scope reservations that are already defined. Link Layer Filter Allow and Deny lists, however, will not be copied over. As a best practice, before running the DHCP Split-Scope Configuration Wizard, create all the necessary reservations on the primary DHCP server scope and manually copy over any Link Layer Filter lists. Ensure that if Link Layer Filtering for either Allow or Deny or both is enabled on the primary server, that the Link Layer Filtering configuration on the secondary DHCP server matches this configuration. To deploy a split-scope configuration—for this example, in an 80/20 split—follow these steps:

Install the DHCP service on two servers. For this example, we will use Server10 as the primary and Server60 as the secondary.

On the primary server, create a new DHCP scope that contains the entire scope range and DHCP options for that scope.

On the secondary server, do not create any scopes.

Open the DHCP server console on the primary server, and expand the server node in the tree pane to reveal the IPv4 and IPv6 nodes.

Add the secondary server to the console by right-clicking on the DHCP node at the top of the tree pane and selecting Add Server.

In the Add Server window, type in the secondary server name or choose it from the managed authorized server list and click OK to complete this task.

After both servers are listed in the console, select and expand the primary server IPv4 node to display the desired IPv4 scope that will be split for this example.

Select and right-click the desired IPv4 scope on the primary DHCP server, select Advanced, and then click on Split-Scope, as shown in Figure 4.

Figure 4. Initiating the DHCP Split-Scope Configuration Wizard.

In the DHCP Split-Scope Configuration Wizard, click Next on the Introduction to DHCP Split-Scope page to continue.

On the Additional DHCP Server page, type in the name of the secondary DHCP server (for this example, Server60), and click Next to continue.

On the Percentage of Split page, the wizard will default to the 80/20 split, which will configure the primary server with 80% of the addresses for lease and exclude the remaining 20%. The secondary server will be configured with 20% of the addresses available for lease and the other 80% will be excluded. Accept the defaults or move the slider to the desired percentage split, and click Next to continue, as shown in Figure 5.

Figure 5. Defining the percentage split of addresses.

On the Delay in DHCP Offer page, define the delay in milliseconds on the secondary server to ensure that the bulk of clients will be serviced by the primary server, and click Next to continue.


Determining how long to delay the secondary DHCP server must be concluded by performing tests on the actual network that will serve the DHCP clients. It is key that the primary server is given ample time to respond and the delay on the secondary is short enough that when the primary server is down, DHCP clients will be acknowledged and serviced by the secondary DHCP server. For our test network, 100 milliseconds was suitable, but on some larger networks, this number might need to be increased.

On the Summary of Split-Scope Configuration page, review the chosen settings and click Finish to configure both of the DHCP servers.

Once the process completes, the status of the configuration changes on both servers is displayed in the window. Review the results and click Close to close the wizard.

Once the wizard is closed, in the tree pane, select and expand the secondary server to expose the IPv4 node.

Expand the IPv4 node to reveal the new scope. Review the scope settings, and if things look good, right-click the scope and click Activate to finalize the split-scope configuration, as shown in Figure 6.

Figure 6. Activating the new split scope on the secondary DHCP server.

Both the primary and secondary DHCP servers configured in a split-scope configuration will honor client scope address reservations, even if the reservation IP address falls within the excluded IP address range. For this to be 100% effective, the reservation will need to be defined on both servers, and the wizard will copy any existing reservations defined on the primary server. Link Layer Filter lists will not be copied over, and any new reservations created on the primary server will need to be manually created on the secondary server scope.

Clustering DHCP Servers

The final redundancy option, and frankly the easiest to configure and maintain, if resources allow, is to deploy a clustered DHCP service. In this configuration, if a single server goes down, the second server in a cluster will take over DHCP operations. This option requires a greater investment in hardware and should be considered only in specific cases in which it is necessary. The biggest benefit for this configuration is that all leases, Link Layer Filter lists, and reservations will be contained with the configuration of the single clustered DHCP server.

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