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Windows Server 2012 : Ensuring DHCP availability (part 1) - Previous approaches to implementing DHCP availability

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5/2/2014 2:47:01 AM

1. Previous approaches to implementing DHCP availability

Traditionally, DHCP server availability has been implemented on Windows Server–based networks using one or more of the following methods:

  • Split scopes This approach involves splitting the IP address pool of a scope between two DHCP servers, typically by assigning the primary server 80 percent of the addresses in the scope and the secondary server the remaining 20 percent of the addresses. That way, if the primary server goes offline for any reason, DHCP clients on the subnet can still respond to lease renewal requests from the secondary server.

  • Server cluster This approach involves using the Failover Clustering feature of Windows Server 2008 or Windows Server 2008 R2 to cluster DHCP servers so that if the primary DHCP server in a cluster fails, the secondary server can take up the slack and continue leasing addresses to clients.

  • Standby server This approach uses a hot standby DHCP server with scopes and options configured identically to your production DHCP server.

Each of the preceding approaches has the following disadvantages, which make them of limited usefulness in ensuring DHCP server availability:

  • The split-scope approach provides limited IP availability during outages. As a result, some clients might not receive addresses during a long-term DHCP server outage. In addition, if your DHCP server scope is currently running at high utilization—which is common for Internet Protocol version 4 (IPv4) networks—splitting the scope might not be feasible.

  • The DHCP server-cluster approach has only one DHCP database located on the cluster shared storage. That means there is a single point of failure for DHCP services on your network. In addition, implementing Failover Clustering requires relatively complex setup processes and maintenance tasks.

  • The hot-standby approach requires both careful configuration of the standby DHCP server and manual intervention on the part of the administrator to ensure the failover transition when your production DHCP server fails or goes offline. There is also additional complexity in this approach when DHCP is configured to automatically update DNS records, as is recommended in an Active Directory environment.

Note

Spanning multiple subnets

Routers usually block DHCP packets from being forwarded from one subnet to another because DHCP packets are broadcast traffic. If an organization’s network consists of multiple subnets, using DHCP for dynamic address assignment requires either of the following:

  • Deploying a DHCP server on each subnet of your network

  • Enabling forwarding of DHCP traffic on your routers by configuring them as DHCP relay agents

Understanding DHCP failover

DHCP failover is a new approach to ensuring DHCP availability that is included in Windows Server 2012. With this approach, two DHCP servers can be configured to provide leases from the same pool of addresses. The two servers then replicate lease information between them, which enables one server to assume responsibility for providing leases to all clients on the subnet when the other server is unavailable. The result of implementing this approach is to ensure DHCP service availability at all times, which is a key requirement for enterprise networks.

The current implementation of DHCP failover in Windows Server 2012 has the following limitations:

  • It only supports using a maximum of two DHCP servers.

  • The failover relationship is limited to IPv4 scopes and subnets.

DHCP server failover can be implemented in two different configurations:

  • Load-sharing mode Leases are issued from both servers equally, which ensures availability and provides load balancing for your DHCP services. (This is the default DHCP server failover configuration.)

  • Hot-standby mode Leases are issued from the primary server until it fails, whereupon the lease data is automatically replicated to the secondary server, which assumes the load.

Load-sharing mode

A typical scenario for implementing the load-sharing approach is when you want to have two DHCP servers at the same physical site. If the site has only a single subnet, all you need to do is enable DHCP failover in its default configuration. If there are multiple subnets, deploy both DHCP servers in the same subnet, configure your routers as DHCP relay agents (or deploy additional DHCP relay agents in subnets), and enable DHCP server failover in its default configuration.

Hot-standby mode

When implementing the hot-standby mode approach, you can configure a DHCP server so that it acts as the primary server for one subnet and as the secondary server for other subnets. One scenario where this approach might be implemented is in organizations that have a central hub site (typically, the data center at the head office) connected via wide area network (WAN) links to multiple remote branch-office sites. Figure 1 shows an example of an organization that has DHCP servers deployed at each branch office and at the head office. Branch-office servers are configured to lease addresses to clients at their branch offices, while the central server leases addresses to clients at the head office. Each branch-office server has a failover relationship with the central server, with the branch office assuming the role as primary and the central server as secondary. That way, if a DHCP server fails at a branch office, the central server can take up the slack for the remote site. For example, the DHCP server at Branch Office A is the primary server for the scope 10.10.0.0/16, while the DHCP server at the Head Office is the secondary for that scope.

Implementing DHCP failover in hot-standby mode in a hub-and-spoke site scenario.
Figure 1. Implementing DHCP failover in hot-standby mode in a hub-and-spoke site scenario.
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