Windows Clustering for High Availability

Windows Clustering for High Availability
Windows Clustering allows management of two or more servers as a single system. It also enables servers running DHCP to provide higher availability, easier manageability, and greater scalability.
Windows Clustering can automatically detect the failure of an application or server and quickly restart it on an alternate server. As a result, users experience only a momentary pause in service. With Windows Clustering, administrators can quickly inspect the status of all cluster resources and easily move workloads to different servers within the cluster, allowing manual load balancing and updates without taking important data and applications offline.
Windows Clustering enables the virtualization of DHCP servers so that if one of the clustered nodes becomes unavailable, the namespace and all of the services provided can reconstitute transparently to the alternate node. This behavior means no changes for the client, which sees the same IP address for the clustered DHCP server.
Without clustering, network administrators might split scopes between servers. If one server becomes unavailable, at least half of the available addresses remain available. Clustering makes more efficient use of IP addresses by removing the need to split scopes. A database stored on a remote disk tracks address assignment and other activity so that if the active cluster becomes unavailable, the second node becomes the DHCP server, with complete knowledge of previous assignments and access to the full scope of addresses. Only one node runs at any given time as a DHCP server; the Windows Clustering remotely stored database provides transparent transition when needed.
Because Windows Clustering works with all clustering-enabled Windows services, the same cluster servers used for DHCP can also support high availability for all other clustering-enabled Windows services.
Automatic Client Configuration
The DHCP client in Windows 2000 and Windows XP has the ability, in the absence of a DHCP server, to configure an IP address and subnet mask automatically when the client starts on a small private network. This feature is called Automatic Private IP Addressing (APIPA). APIPA assigns a unique address from the range 169.254.0.1 through 169.254.255.254 with the subnet mask of 255.255.0.0.
The DHCP client service uses a two-step process to configure the client with an IP address and other configuration information. Upon installation, the client attempts to locate a DHCP server and obtain a configuration from it. Most TCP/IP networks use DHCP servers that are configured to dispense information to clients on the network. For Windows–based operating systems, if the first attempt to locate a DHCP server fails, the DHCP client configures itself with a selected IP address and APIPA address.
If the DHCP client has previously obtained a lease from the DHCP server and the lease has not expired, the client tries to renew any unexpired lease with the DHCP server. If the client fails to locate any DHCP server, it attempts to ping the default gateway listed in the lease. If this action is successful, the client assumes it has not moved to a different network and uses that lease. The client then seeks to automatically renew the lease when half of the lease time has expired.
If the attempt to ping the default gateway fails, the client assumes that it moved to a network that has no DHCP services available, and it configures itself with an APIPA address. It then automatically keeps trying to locate a DHCP server every five minutes.
Alternate Client Configuration
With DHCP client alternate configuration, administrators can easily move a computer among two or more networks, one configured with static IP addresses and one or more configured with DHCP. Alternate configuration provides simplified computer migration (for example, a laptop) among networks without requiring reconfiguration of network adapter parameters, such as IP address, subnet mask, default gateway, preferred and alternate DNS servers, and WINS servers.
Configuration Options
Table 2 shows options that are available for configuring TCP/IP properties for a local area network (LAN) connection.
Table 2. Configuring TCP/IP Properties for LAN Connections
Option
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Description
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Static IP address configuration
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When you click Use the following IP address, you can provide values for static IP address settings, such as IP address, subnet mask, default gateway, preferred and alternate DNS servers, and WINS servers. However, if you click Obtain an IP address automatically to change the configuration of your network adapter to a DHCP client configuration, all static IP address settings are lost. Additionally, if you move the computer and configure it for another network, when you return to the original network you must reconfigure the computer with the original static IP address settings.
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Dynamic IP address configuration without alternate configuration
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When you click Obtain an IP address automatically, your computer acts as a DHCP client and obtains IP address, subnet mask, and other configuration parameters from your network DHCP server. If the DHCP server is unavailable, the network adapter is configured using IP autoconfiguration.
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Dynamic IP address configuration with alternate configuration
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When you click Obtain an IP address automatically, click the Alternate Configuration tab, and type an alternate configuration, you can move your computer from one statically configured network (such as a home network) to one or more dynamically configured networks (such as a corporate network) without changing any settings. If a DHCP server is unavailable (for example, when your computer is connected to your home network), the network adapter is automatically configured with your alternate configuration, and the computer functions correctly on the network. When you move the computer back to the dynamically configured network and the DHCP server is available, the network adapter is automatically configured with the dynamic configuration assigned by the DHCP server. The alternate configuration is used only when the DHCP client cannot locate a DHCP server.
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DHCP Server Discover Attempts
If you use DHCP without an alternate configuration and the DHCP client cannot locate a DHCP server, IP autoconfiguration is used to configure the network adapter. The DHCP client continues to attempt to discover a DHCP server on the network every five minutes. If a DHCP server is found, the network adapter is assigned a valid DHCP IP address lease.
If you use DHCP with an alternate configuration, and the DHCP client cannot locate a DHCP server, the alternate configuration is used to configure the network adapter. Normally, there are no additional discovery attempts. However, a DHCP server discovery attempt will occur in the following cases:
The network adapter is disabled and then enabled again.
Media (such as network cabling) is disconnected and then reconnected.
The TCP/IP settings for the adapter are changed, and DHCP remains enabled after these changes.
If a DHCP server is found, the network adapter is assigned a valid DHCP IP address lease.
Command-Line Management
The Netsh DHCP component offers a command-line tool that helps with the administration of DHCP servers and provides an equivalent alternative to console-based management. Command-line management is useful in the following situations:
When managing DHCP servers in wide area networks (WANs), you can more effectively manage across slow-speed network links by using commands in interactive mode at the Netsh command prompt.
When managing a large number of DHCP servers, you can script and automate recurring administrative tasks by using batch mode.
For a comprehensive Netsh DHCP command reference, including syntax, parameters, and examples, see the topic titled “Netsh commands for DHCP” in Windows Server 2003 Help and Support Center.
DHCP Overview
DHCP is derived from the Internet standard BOOTP (RFCs 951 and 1084), which allows dynamic assignment of IP addresses as well as remote booting of diskless workstations. In addition to supporting the dynamic assignment of IP addresses, DHCP supplies all configuration data required by TCP/IP, plus additional data required for specific services.
As noted, this functionality simplifies tasks for the network administrator, who can now manually configure just one computer—the DHCP server. Whenever a new computer starts on a network segment that is served by the DHCP server (or an existing computer is restarted), the computer asks for a unique IP address and the DHCP server assigns one from the pool of available addresses.
As Figure 2 shows, this process requires only four steps:
The DHCP client asks for an IP address (a DHCP Discover message).
The DHCP Server offers an address (a DHCP Offer message).
The DHCP client accepts the offer and requests the address (a DHCP Request message).
The DHCP Server officially assigns the address to the client (a DHCP Acknowledge message).
Figure 2. DHCP Automates the Assignment of IP Addresses
The DHCP server places an administrator-defined time limit, called a lease, on the address assignment. Halfway through the lease period, the DHCP client requests a lease renewal, and the DHCP server extends the lease. As a result, when a computer stops using its assigned IP address (for example, upon relocation to another network segment), the lease expires and the address returns to the pool for reassignment.
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