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Step-by-Step: Configure DHCP for Failover

Published: February 29, 2012

Updated: February 29, 2012

Applies To: Windows Server 2012



Dynamic Host Configuration Protocol (DHCP) failover in Windows Server 2012 is a new method for ensuring continuous availability of DHCP service to clients.

This guide provides step-by-step instructions for deploying DHCP failover in a test lab using two server computers and one client computer. Software and hardware requirements are provided, as well as an overview of DHCP failover.

ImportantImportant
The following instructions are for configuring a test lab using the minimum number of computers. Individual computers are needed to separate the services provided on the network and to clearly show the desired functionality. This configuration is neither designed to reflect best practices nor does it reflect a desired or recommended configuration for a production network. The configuration, including IP addresses and all other configuration parameters, is designed only to work on a separate test lab network.

In Windows Server® 2008 R2, there are two high availability options available for DHCP Server deployment. Each of these options is associated with some challenges.

  1. DHCP in a Windows failover cluster. This option places the DHCP server in a cluster with an additional server configured with the DHCP service that assumes the load if the primary DHCP server fails. The clustering deployment option uses a single shared storage. This makes the storage a single point of failure, and requires additional investment in redundancy for storage. In addition, clustering involves relatively complex setup and maintenance.

  2. Split scope DHCP. Split scope DHCP uses two independent DHCP servers that share responsibility for a scope. Typically 70% of the addresses in the scope are assigned to the primary server and the remaining 30% are assigned to the backup server. If clients cannot reach the primary server then they can get an IP configuration from the secondary server. Split scope deployment does not provide IP address continuity and is unusable in scenarios where the scope is already running at high utilization of address space, which is very common with Internet Protocol version 4 (IPv4).

DHCP failover in Windows Server 2012 enables administrators to deploy a highly resilient DHCP service to support a large enterprise without the challenges of the options discussed earlier. The main goals of the feature are the following:

  • Provide DHCP service availability at all times on the enterprise network.

  • If a DHCP server is no longer reachable, the DHCP client is able to extend the lease on its current IP address by contacting another DHCP server on the enterprise network.

The DHCP server failover feature provides the ability to have two DHCP servers provide IP addresses and option configuration to the same subnet or scope, providing for continuous availability of DHCP service to clients. The two DHCP servers replicate lease information between them, allowing one server to assume responsibility for servicing of clients for the entire subnet when the other server is unavailable. It is also possible to configure failover in a load-balancing configuration with client requests distributed between the two servers in a failover relationship.

DHCP failover in Windows Server 2012 provides support for a maximum of two DHCP servers, and the failover relationship is limited to IPv4 scopes and subnets. Network nodes using Internet Protocol version 6 (IPv6) typically determine their own IPv6 address using stateless IP auto configuration. In this mode, the DHCP server delivers only the DHCP option configuration, and the server does not maintain any lease state information. A high availability deployment for stateless DHCPv6 is possible by simply setting up two servers with identical option configuration. Even in a stateful DHCPv6 deployment, the scopes do not run under high address utilization, which makes split scope a viable solution for high availability.

Administrators can deploy DHCP servers running Windows Server 2012 as failover partners in either hot standby mode or load sharing mode.

In hot standby mode, two servers operate in a failover relationship where an active server is responsible for leasing IP addresses and configuration information to all clients in a scope or subnet. The secondary server assumes this responsibility if the primary server becomes unavailable. A server is primary or secondary in the context of a subnet. For instance, a server that has the role of a primary for a given subnet could be a secondary server for another subnet.

Hot standby mode of operation is best suited to deployments where a central office or data center server acts as a standby backup server to a server at a remote site, which is local to the DHCP clients (ex: hub and spoke deployment). In such deployments, it is undesirable to have a remote standby server service any clients unless the local DHCP server becomes unavailable. The figure below is an example of a hub and spoke deployment.

Host Standby Mode

In a load sharing mode deployment, which is the default mode of operation, the two servers simultaneously serve IP addresses and options to clients on a given subnet. The client requests are load balanced and shared between the two servers.

The load sharing mode of operation is best suited to deployments where both servers in a failover relationship are located at the same physical site. Both servers respond to DHCP client requests based on the load distribution ratio configured by the administrator. See the following examples.

In the following example, two DHCP servers simultaneously provide IP addressing to clients on a single subnet.

Load sharing single subnet

In the following example, two DHCP servers simultaneously provide IP addressing to clients on multiple subnets.

Load sharing multiple subnets

This test lab demonstrates new DHCP functionality in Windows Server 2012. Two server computers and one client computer are used. See the following figure.

DHCP lab

Two server computers and one client computer are required to complete the test lab.

The following are required components of the test lab:

  1. The product disc or other installation media for Windows Server 2012.

  2. Two computers that meet the minimum hardware requirements for Windows Server 2012.

  3. The product disc or other installation media for Windows® 8.

Number of servers: DHCP failover is not supported for more than two DHCP servers. The failover relationship is always comprised of two DHCP servers.

Domain membership: In this guide, DHCP servers are domain member servers. You can also configure DHCP failover on workgroup computers.

Time synchronization: For DHCP failover to function correctly, time must be kept synchronized between the two servers in a failover relationship. Time synchronization can be maintained by deployment of the Network Time Protocol (NTP) or any alternate mechanism. When the failover configuration wizard is run, it will compare the current time on the servers being configured for failover. If the time difference between the servers is greater than one minute, the failover setup process will halt with a critical error instructing the administrator to synchronize the time on the servers.

The following procedures are used to configure computers for the demonstration portion of the test lab:

  1. Configure DHCP1: DHCP1 is a domain controller, DNS server, and DHCP server for the contoso.com Active Directory domain.

    ImportantImportant
    Domain controller and DNS server roles are not required for DHCP failover. These roles are installed on DHCP1 to mimic a domain environment. DHCP failover can also be configured on a workgroup computer (not demonstrated in this test lab).

  2. Configure DHCP2: DHCP2 is a DHCP server and domain member computer.

  3. Configure Client1: Client1 is a DHCP client computer.

DHCP1 is a computer running Windows Server 2012, providing the following services:

  • A domain controller for the contoso.com Active Directory domain.

  • An authoritative DNS server for the contoso.com DNS zone.

  • A DHCP server.

Initial configuration of DHCP1 consists of the following steps:

noteNote
It is not necessary to configure a DHCP scope on DHCP1. A DHCP scope will be configured automatically when a failover relationship is created with DHCP2.

  1. Start your computer using the Windows Server 2012 product disc or other digital media.

  2. When prompted, enter a product key, accept license terms, configure clock, language, and regional settings, and provide a password for the local Administrator account.

  3. Press Ctrl+Alt+Delete and sign-in using the local Administrator account.

  4. If you are prompted to enable Windows Error Reporting, click Accept.

  5. Click Start, type ncpa.cpl, and then press ENTER. The Network Connections control panel will open.

    TipTip
    The previous step demonstrates new functionality in Windows Server 2012 that enables you to search and run applications, settings, and files by clicking Start and then typing a search term. You can also open the Network Connections control panel by clicking next to Wired Ethernet Connection in Server Manager using the Local Server view. For more information, see Common Management Tasks and Navigation in Windows Server 2012 (http://go.microsoft.com/fwlink/p/?LinkId=242147).

  6. In Network Connections, right-click Wired Ethernet Connection and then click Properties.

  7. Double-click Internet Protocol Version 4 (TCP/IPv4).

  8. On the General tab, choose Use the following IP address.

  9. Next to IP address type 10.0.0.1 and next to Subnet mask type 255.255.255.0. It is not necessary to provide an entry next to Default gateway.

  10. Next to Preferred DNS server, type 10.0.0.1.

  11. Click OK twice, and then close the Network Connections control panel.

DHCP1 will serve as a domain controller, DNS server, and DHCP server for the contoso.com Active Directory domain.

  1. The Server Manager Dashboard is displayed by default. In the navigation pane, click Configure this local server.

  2. Under PROPERTIES, click the name next to Computer name. The System Properties dialog box will open.

  3. On the Computer Name tab, click Change and then type DHCP1 under Computer name.

  4. Click OK twice, and then click Close.

  5. When you are prompted to restart the computer, click Restart Now.

  6. After restarting the computer, sign-in using the local Administrator account.

  7. In Server Manager, under Configure this local server, click Add Roles and Features.

  8. In the Add Roles and Features Wizard, click Next three times, and then on the Select server roles page select the Active Directory Domain Services checkbox.

  9. When you are prompted to add required features, click Add Features.

  10. Select the DHCP Server checkbox.

  11. When you are prompted to add required features, click Add Features.

  12. Select the DNS Server checkbox.

  13. When you are prompted to add required features, click Add Features.

  14. Click Next five times, and then click Install.

  15. Wait for the installation process to complete, verify on the Installation progress page that Configuration required. Installation succeeded on DHCP1 is displayed, and then click Close.

  16. Click the Notification flag and then click Promote this server to a domain controller. See the following example.

    Notification flag
  17. In the Active Directory Domain Services Configuration Wizard, on the Deployment Configuration page, choose Add a new forest and then next to Root domain name, type contoso.com.

  18. Click Next, and then on the Domain Controller Options page, under Type the Directory Services Restore Mode (DSRM) password, type a password next to Password and Confirm password. Confirm that Domain Name System (DNS) server and Global Catalog (GC) are selected, and then click Next.

  19. Click Next four times, verify that All prerequisite checks passed successfully is displayed, and then click Install.

  20. The computer will restart automatically to complete the installation process.

  21. Sign in using the local Administrator account.

A domain administrator account is required to configure settings in the test lab.

TipTip
You can use the CONTOSO\Administrator account in this test lab and skip creation of a domain administrator account if desired. This account has domain administrator privileges, and other privileges. However, it is a best practice to disable or rename this account. For more information, see Active Directory Best Practices(http://go.microsoft.com/fwlink/p/?LinkID=243071).

  1. On the Server Manager menu bar, click Tools, and then click Active Directory Users and Computers.

  2. In the Active Directory Users and Computers console tree, double-click contoso.com, right-click Users, point to New, and then click User.

  3. In the New Object – User dialog box, type user1 under User logon name and next to Full name, then click Next.

  4. Next to Password and Confirm password, type a password for the user1 account.

  5. Clear the checkbox next to User must change password at next logon, select the Password never expires checkbox, click Next, and then click Finish.

  6. Double-click user1 and then click the Member Of tab.

  7. Click Add, type domain admins under Enter the object names to select, click OK twice, and then close the Active Directory Users and Computers console.

  8. Click Start, click Administrator, and then click Sign out.

  9. Sign in to the computer using the user1 credentials by clicking the left arrow next to CONTOSO\Administrator and then clicking Other user.

So that DHCP1 can be configured as a failover partner, it must be authorized in Active Directory.

  1. On the Server Manager menu, click Tools and then click DHCP.

  2. In the DHCP console, right-click dhcp1.contoso.com and then click Authorize.

  3. Refresh the DHCP console and verify that DHCP1 was successfully authorized.

  4. On the Server Manager menu, click the Notification flag and then click Complete DHCP configuration.

  5. In the DHCP Post-Install configuration wizard, click Commit and then click Close.

DHCP2 is a computer running Windows Server® 2012, providing the following services:

  • A DHCP server.

Initial configuration of DHCP2 consists of the following steps:

During the demonstration portion of the test lab, DHCP2 will be used to create a failover relationship with DHCP1.

TipTip
The procedure below is identical to the steps used to install the operating system and configure TCP/IP on DHCP1, with the exception that DHCP2 is configured with an IP address of 10.0.0.2.

  1. Start your computer using the Windows Server 2012 product disc or other digital media.

  2. When prompted, enter a product key, accept license terms, configure clock, language, and regional settings, and provide a password for the local Administrator account.

  3. Press Ctrl+Alt+Delete and sign-in using the local Administrator account.

  4. If you are prompted to enable Windows Error Reporting, click Accept.

  5. In the Server Manager navigation pane, click Local Server and then click the IP address next to Wired Ethernet Connection. The Network Connections control panel will open.

  6. In Network Connections, right-click Wired Ethernet Connection and then click Properties.

  7. Double-click Internet Protocol Version 4 (TCP/IPv4).

  8. On the General tab, choose Use the following IP address.

  9. Next to IP address type 10.0.0.2 and next to Subnet mask type 255.255.255.0. It is not necessary to provide an entry next to Default gateway.

  10. Next to Preferred DNS server, type 10.0.0.1.

  11. Click OK twice, and then close the Network Connections control panel.

DHCP2 is a domain member server running the DHCP Server role service.

  1. In the Server Manager navigation pane, click Local Server and then click the name next to Computer name. The System Properties control panel will open.

  2. On the Computer Name tab, click Change and then type DHCP2 under Computer name.

  3. Under Member of, select Domain, type contoso.com, and then click OK.

  4. When you are prompted to provide credentials to join the domain, enter the credentials for the user1 account that was created previously and then click OK.

  5. Confirm that computer name and domain changes were successful, click OK twice, and then click Close.

  6. When you are prompted to restart the computer, click Restart Now.

  7. After restarting the computer, sign-in using the CONTOSO\user1 account.

  8. In Server Manager, under Configure this local server, click Add Roles and Features.

  9. In the Add Roles and Features Wizard, click Next three times, and then on the Select server roles page select the DHCP Server checkbox.

  10. When you are prompted to add required features, click Add Features.

  11. Click Next three times, and then click Install.

  12. Wait for the installation process to complete, verify on the Installation progress page that Configuration required. Installation succeeded on DHCP2.contoso.com is displayed, and then click Close.

  1. On the Server Manager menu bar, click Tools and then click DHCP. THE DHCP console opens.

  2. In the DHCP console tree, navigate to IPv4. Right-click IPv4 and then click New Scope. The New Scope Wizard opens.

  3. Click Next and then type a name for the new scope next to Name (ex: Contoso-scope1).

  4. Click Next and then in IP Address Range, type 10.0.0.1 next to Start IP address, type 10.0.0.254 next to End IP address, and type 24 next to Length. The value of Subnet mask will change automatically to 255.255.255.0.

  5. Click Next, and then in Add Exclusions and Delay type 10.0.0.1 under Start IP address, type 10.0.0.10 under End IP address, and then click Add. This allows the first ten IP addresses in the 10.0.0.0/24 subnet to be used for static addressing of servers on the network.

  6. Click Next and then in Lease Duration under Limited to enter 0 Days, 0 Hours, and 2 Minutes. This very short lease duration will simplify the DHCP failover demonstration.

  7. Click Next three times, and then in Domain Name and DNS Servers, verify that the Parent domain is contoso.com and 10.0.0.1 is listed as the only DNS server.

  8. Click Next twice, and then in Activate Scope select Yes, I want to activate this scope now.

  9. Click Next, and then click Finish.

  10. In the DHCP console tree, right-click dhcp2.contoso.com, and then click Authorize.

  11. Refresh the view in the DHCP console and verify that DHCP2 is authorized and that the Contoso-scope1 is active.

    Note: To review scopes on the current server using Windows PowerShell, right-click Windows PowerShell, click Run as Administrator, click Yes in the User Account Control alert that appears, and then type the following command at the Windows PowerShell prompt, and then press ENTER.

    get-dhcpserverv4scope
    
    PS C:\Windows\system32> get-dhcpserverv4scope
    
    ScopeId         SubnetMask      Name           State    StartRange      EndRange        LeaseDuration
    -------         ----------      ----           -----    ----------      --------        -------------
    10.0.0.0        255.255.255.0   Contoso-scope1 Active   10.0.0.1        10.0.0.254      00:02:00

Client1 is a computer running Windows® 8 that is acting as a DHCP client.

Configuration of Client1 consists of the following steps:

During the demonstration portion of the test lab, Client1 will be used as a DHCP client.

  1. Start your computer using the Windows 8 product disc or other digital media.

  2. When prompted, enter a product key and accept license terms.

  3. When prompted to enter a computer name, type Client1 and click Next.

  4. Click Use express settings.

  5. On the Sign in to your PC page, click Don’t want to sign in with a Microsoft account and then click Local account.

  6. Next to User name, type user1, enter a password and password hint, and then click Finish.

The DHCP failover demonstration on Client1 makes use of Windows PowerShell to verify DHCP lease information. To make Windows PowerShell more easily accessible, it will be pinned to the taskbar.

  1. The Start menu is displayed by default. If Start is not displayed, move the mouse cursor to the lower left corner of the screen until Start is displayed, and then click Start.

  2. Type power and then under Results for “power” right-click Windows PowerShell and then click Pin to taskbar.Confirm that Windows PowerShell is pinned to the taskbar.

noteNote
Client1 can also be joined to the contoso.com domain, however this is not required to complete the test lab.

For the DHCP failover demonstration portion of the test lab, a failover relationship will be created using DHCP1 and DHCP2.

A demonstration of DHCP failover on Windows Server 2012 consists of the following procedures:

  1. Configure a failover relationship

  2. View or edit properties of the failover configuration

  3. Edit properties of the failover relationship and demonstrate load balancing

  4. Edit properties of the failover relationship and demonstrate hot standby mode

First, configure a failover relationship using DHCP1 and DHCP2.

  1. On DHCP2, open the DHCP console, right-click the Contoso-scope1 DHCP scope and then click Configure failover.

    Configure failover
  2. In the Configure Failover wizard, click Next.

  3. In Specify the partner server to use for failover, next to Partner Server, type dhcp1.contoso.com and then click Next.

  4. In Create a new failover relationship, type a name next to Relationship Name, or accept the default name that is displayed (dhcp2.contoso.com-dhcp1.contoso.com).

  5. Type a shared secret for this failover relationship next to Shared Secret (ex: secret).

  6. Change the value next to Maximum Client Lead Time to 0 hours and 1 minute.

    ImportantImportant
    The Maximum Client Lead Time (MCLT) is additional time provided to a DHCP client after expiration of a DHCP lease. The MCLT is transmitted from the primary to the secondary server in the CONNECT message, and is the maximum amount of time that one server can extend a lease for a client beyond the time known by the partner server.

    The 1 minute MCLT value used here is for test lab purposes only, to prompt lease renewal by the client. In a production environment, you should use a longer MCLT, such as 1 hour.

  7. Review the options available in the drop-down menu next to Mode. You can choose Load balance or Hot standby. By default, Load balance mode is chosen.

    Configure Failover
  8. Click Next and then click Finish.

  9. Verify that failover configuration was successful, and then click Close.

    Configure Failover
  10. On DHCP1, refresh the DHCP console and verify that the same DHCP scope configuration that is present on DHCP2 is now present on DHCP1.

After you configure a failover relationship on a DHCP server, details for the failover relationship are displayed in the DHCP console.

  1. On DHCP1 or DHCP2, in the DHCP console, right-click the Contoso-scope1 DHCP scope and then click Properties.

  2. Click the Failover tab and review the information displayed. Verify that Normal is displayed next to State of this Server and also next to State of Partner Server.

    Scope Properties
  3. Note that you can edit or delete the failover relationship.

    IPv4 Properties
  4. Click Edit and review properties of the failover relationship that are available to edit.

  5. Leave the dialog box open for the following procedure.

To demonstrate dynamic load balancing properties of the failover relationship, the load balancing percentage will be changed for an active DHCP scope.

  1. On Client1, click Windows PowerShell and type the following command.

    ipconfig /all
    
  2. In the command output, note the DHCP server that is currently supplying an IP address configuration to Client1. The IP address of the DHCP server is displayed next to DHCP Server.

  3. In the View/Edit Failover Relationship dialog box DHCP1 or DHCP2 that was opened in the previous procedure, change the values under Load Balance Mode next to Local Server and Partner Server so that 100% is assigned to the DHCP server that is currently not supplying an IP address to Client1. The server that is currently supplying an IP address to Client1 will have a value assigned of 0%.

    Load Balance Percentage
  4. Click OK twice, wait until the current DHCP lease is expired on Client1, and then type ipconfig /all again at the Windows PowerShell prompt.

  5. Note that the DHCP server that is supplying an IP address configuration to Client1 has changed to the server that you assigned 100% weight in load balancing mode.

To demonstrate hot standby mode, the DHCP Server service on one of the failover partners will be stopped.

  1. On DHCP1 or DHCP2, in the DHCP console, right-click the Contoso-scope1 DHCP scope and then click Properties.

  2. Click the Failover tab.

  3. Click Edit and then choose Hot Standby Mode.

  4. Depending on which DHCP server you are configuring, the local server will be assigned either Active or Standby status. The status is displayed next to Role of this server.

    Hot Standby Mode
    TipTip
    The server that is designated to be Active in hot standby mode is the server that you used to create the failover relationship.

  5. Click OK twice and then wait 2 minutes for the DHCP lease on Client1 to renew.

  6. On Client1, type ipconfig /all at the Windows PowerShell prompt and verify that the server that is assigned as Active is supplying an IP addresses configuration to Client1.

  7. In the DHCP console on the DHCP server that is marked as Active for the hot standby failover relationship and is currently supplying an IP address to Client1, right-click the server name, point to All Tasks, and then click Stop.

  8. Verify that the DHCP service is stopped on the active DHCP server.

    Server down
  9. Wait for the DHCP lease to renew on Client1, type ipconfig /all at the Windows PowerShell prompt, and verify that the standby DHCP server is supplying an IP address to Client1.

DHCP failover provides high availability of DHCP services without the challenges of clustering or split scope DHCP. Benefits of DHCP failover include:

  1. Simple: A wizard is provided to create DHCP failover relationships between DHCP servers. The wizard automatically replicates scopes and settings from the primary server to the failover partner.

  2. Flexible: DHCP failover can also be configured for load balancing, with client requests distributed between both DHCP servers in a failover relationship based on the values you choose.

  3. Seamless: DHCP servers share lease information, allowing one server to assume responsibility for servicing of clients if the other server is unavailable. DHCP clients can keep the same IP address when a lease is renewed, even if the lease is issued by a different DHCP server.

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