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.
