Logo - tutorial.programming4.us
Windows XP
Windows Vista
Windows 7
Windows Azure
Windows Server
Windows Phone
Windows Server

Windows Server 2012 : Enhancing DHCP Reliability - Implementing Redundant DHCP Services

12/8/2014 8:44:48 PM

DHCP is a critical network service and should be treated as such. Building redundancy into DHCP services has been a challenge for years, and with each release of Windows Server, DHCP redundancy options get better. Windows Server 2012 DHCP server service is no different. The biggest improvement for the DHCP server service is the now built-in failover option, but that is not the only option. The following sections detail historic and current DHCP redundancy options that can be leveraged to improved DHCP reliability.

DHCP Split Scopes

Historically, when administrators required DHCP redundancy, DHCP was deployed on a failover cluster or multiple DHCP servers were deployed with split-scope configuration. A split scope is simply the division of the entire pool of DHCP IP addresses across multiple servers. You can split the scope in various ways, as follows:

50/50 split-scope configuration—The 50/50 split-scope configuration, as the name indicates, takes half of the DHCP IP address pool, and a scope is created on each server with nonoverlapping addresses. This can work well if both DHCP servers answer at the same time when a DHCP request comes across the network or if some hardware or software load balancer manages the requests. The challenge arises if all or most of the IP addresses will be leased. When a DHCP server configured with only half of the IP addresses is out of leases, that does not stop it from answering DHCP client requests, and clients can end up without an IP address, even if the second server still has available IP addresses for leasing.

The 80/20 split-scope configuration—The 80/20 split scope configuration is the most ideal configuration for Windows Server 2012 and Windows Server 2008 R2 DHCP servers. With the release of Windows Server 2008 R2 and included with Windows Server 2012, DHCP scope settings now allow for a delayed response interval configuration. With an 80/20 split, the server configured with 20% of the addresses is also configured with a delayed response to DHCP client requests. This results in the 20% server becoming more of a backup DHCP server that will be used only in the event of an issue with the primary DHCP server.

The 100/100 split-scope configuration—The 100/100 split-scope option can be the best configuration, but it requires that 200% of the necessary IP addresses are available to the DHCP IP address pool. For example, if a network will support up to 200 DHCP clients, the DHCP range requires at least 400 IP addresses in the entire DHCP pool. This, of course, is not available in the standard Class C network configuration, so networking changes may be required for this type of configuration to be implemented. With this configuration, no delayed response is required, and clients can get an IP address from either server as required.

Windows Server 2012 Delay Configuration Setting

Windows Server 2012 includes a response delay configuration on the DHCP scope settings. This enables administrators to implement redundant DHCP scope configurations across the network, with different version of DHCP servers. To implement a delayed response to a DHCP server on a particular scope, open the scope properties on the desired DHCP server scope and display the Advanced tab. Near the bottom, under Delay Configuration, enter the delay interval in milliseconds and click OK to save the setting, as shown in Figure 1. Administrators must test the amount of delay required to get the desired response time from the redundant or secondary DHCP server.


Figure 1. Implementing a delay configuration on a DHCP scope.

Windows Server 2012 Split Scope Versus Failover

Windows Server 2012 includes a Split-Scope Wizard and a feature called DHCP failover. The Split-Scope Wizard enables administrators to set up a scope across two DHCP servers, including defining the delay configuration, but leases and reservations are not shared or in sync across servers. Furthermore, DHCP clients get a different IP address from each DHCP server the clients obtain leases from. Windows Server 2012 failover is a single DHCP scope configured across two servers. Lease and reservation information is kept in sync across the servers.

DHCP Split-Scope Configuration Wizard

When a split scope configuration is desired, the DHCP administrator can run the DHCP Split-Scope Wizard to simplify the process. If reservations are already created, the Split-Scope Wizard replicates these reservations, but will not keep these reservations in sync after the split-scope is created. To create a split-scope across two DHCP servers using the wizard, follow these steps.

1. Install the DHCP server service on at least two DHCP servers and authorize them both.

2. Log on to the primary DHCP server and open the console. Expand the IPv4 node and create the desired scope as outlined previously in this chapter.

3. Once the scope is created, right-click the scope in the tree pane and select Advanced and select Split-Scope. The DHCP Split-Scope Wizard opens.

4. On the Welcome page of the DHCP Split-Scope Wizard window, click Next to continue.

5. On the Additional DHCP Server page, click the Add Server button to show the list of authorized DHCP servers. Select the desired server or type the name in and click OK to return to the wizard windows.

6. Once the additional DHCP server is listed, click Next to continue.

7. On the Percentage of Split page, the default is an 80/20 split, with the 20% going to the additional server. If this is the desired configuration, click Next to continue, as shown in Figure 2.


Figure 2. Configuring the percentage of split IP addresses in the Split-Scope Wizard.

8. On the Delay in DHCP Offer page, enter 0 for the Host DHCP Server and enter the desired delay for the Additional DHCP Server (for example, 200 milliseconds), and then click Next.

9. On the Summary page, review the configuration. If everything looks correct, click Finish to commit the changes and update the scope on both DHCP servers.

10. When the process completes, connect to each of the DHCP servers and verify the scope settings. If the scopes are correct, activate the new scope on the additional DHCP server and on the host DHCP server if not already activated.

This completes the DHCP split-scope configuration task.

Other -----------------
- Windows Server 2012 : Enhancing DHCP Reliability - DHCP Network Access Protection Integration
- Windows Server 2012 : Enhancing DHCP Reliability - DHCP Name Protection, DHCP and Dynamic DNS Configuration
- Windows Server 2012 : Enhancing DHCP Reliability - Link-Layer Filtering, DHCP Reservations
- Exploring DHCP Changes in Windows Server 2012 : Migrating DHCP Services from 2008 R2 to Windows Server 2012, derstanding DHCP Client Alternate Network Capability
- Exploring DHCP Changes in Windows Server 2012 : Migrating DHCP Servers Using Windows Server Migration Tools
- Sharepoint 2013 : The Office Web Applications for Sharepoint - Preparing the Server and Installing OWA via the GUI
- Sharepoint 2013 : The Office Web Applications for Sharepoint - Topology
- Sharepoint 2013 : The Office Web Applications for Sharepoint - Mobile Device Support
- Sharepoint 2013 : The Office Web Applications for Sharepoint - Desktop Enhancements
- Sharepoint 2013 : The Office Web Applications for Sharepoint - Licensing and Versions
Top 10
- Microsoft Exchange Server 2013 : Working with cmdlets (part 2) - Understanding cmdlet errors, Using cmdlet aliases
- Microsoft Exchange Server 2013 : Working with cmdlets (part 1) - Using Windows PowerShell cmdlets, Using cmdlet parameters
- Microsoft Exchange Server 2013 : Using Windows PowerShell (part 2) - Running and using cmdlets, Running and using other commands and utilities
- Microsoft Exchange Server 2013 : Using Windows PowerShell (part 1) - Running and using Windows PowerShell
- Troubleshooting Stop Messages : Being Prepared for Stop Errors - Prevent System Restarts After a Stop Error
- Troubleshooting Stop Messages : Memory Dump Files (part 3) - Using Memory Dump Files to Analyze Stop Errors - WinDbg Debugger
- Troubleshooting Stop Messages : Memory Dump Files (part 2) - Using Memory Dump Files to Analyze Stop Errors - Using Problem Reports And Solutions
- Troubleshooting Stop Messages : Memory Dump Files (part 1) - Configuring Small Memory Dump Files, Configuring Kernel Memory Dump Files
- Troubleshooting Stop Messages : Stop Message Overview - Identifying the Stop Error, Finding Troubleshooting Information
- Deploying IPv6 : Planning for IPv6 Migration - Understanding ISATAP, Migrating an Intranet to IPv6