Ensuring DHCP Server Availability
By Mitch Tulloch
DHCP makes the life of a network administrator easier by automating the process of
assigning IP address information to clients on your network. (Servers should generally
have static addresses instead of dynamic ones.) However, using DHCP on your
network introduces a point of failure--for what if your DHCP server suddenly dies?
With Windows 2000 and later this can be a problem because if the machine is
configured as a DHCP client and its lease expires, it tries to renew its lease by
communicating with a DHCP server on the network. If no DHCP server can be found,
however, the client uses Automatic Private IP Addressing (APIPA) to auto-assign
itself an IP address from the reserved address range 169.254.0.0
169.254.255.255. And when this happens, the client can no longer communicate
with other machines on the network unless they have addresses from the same range.
This is a good reason why you should disable APIPA on your Windows 2000 or
Windows XP client machines, if you plan on using DHCP on your network.
Because DHCP servers are so important for ensuring network communications,
however, it's also a good idea to take measures to ensure their availability. To begin
with, this means using high-availability hardware platforms that have RAID 0+1, hot
swappable drives and power supplies, and other forms of hardware fault tolerance for
your DHCP servers. This way if a power supply dies or a drive fails, you can have
your DHCP server up and running again in minutes. The moral is don't skimp on
hardware costs for servers that your whole network depends on.
Clustering might seem like another option, but you have to be careful here. A
Windows 2000/2003 clustered DHCP server is considerably more complicated to set
up than a standard DHCP server, and the hardware requirements are higher as well.
Additionally, not many companies can afford the resources to dedicate a server cluster
to the simple job of leasing IP addresses for their network.