Dynamic Host Configuration Protocol

DHCP Service Installation

Before you install a DHCP server, identify the following:

The hardware and storage requirements for the DHCP server.
Which computers you can immediately configure as DHCP clients for dynamic TCP/IP configuration and which computers you should manually configure with static TCP/IP configuration parameters.
The DHCP option types and the option values that will be predefined for the DHCP clients.

DHCP Server Location Use the physical characteristics of your LAN or WAN infrastructure and not the logical groupings defined by Windows 2000 domains and your Active Directory structure. When subnets are connected by routers that support BOOTP relay agents, DHCP servers are not required on every subnet.

Also, DHCP servers can be administered remotely from a computer running Windows 2000 and DHCP Manager.

Resources Compile a list of requirements, including:

The number and types of computers that need to be supported.
Interoperability with existing systems, including your requirements for mission-critical accounting, personnel, and similar information systems.
Hardware support and related software compatibility, including routers, switches, and other types of servers.
Network monitoring software, such as Net Monitor (provided with Windows 2000).

Process Isolation Isolate the areas of the network where processes must continue uninterrupted, and then target these areas for the last stages of implementation.

Logical Subnet Planning Review the geographic and physical structure of the network to determine the best plan for defining logical subnets as segments of the intranet.

Test Phases Define the components in the new system that require testing, and then develop a phased plan for testing and adding components. For example, the plan could define the order of types of computers to be phased in, including Windows 2000 servers and workstations, Microsoft remote access servers and clients, Windows for Workgroups computers, and MS-DOS clients.

Create a pilot and a second test phase, including tuning the DHCP and WINS server-client configuration for efficiency. This task includes determining strategies for backup servers and for partitioning the address pool at each server for local vs. remote clients.
Document all architecture and administration issues for network administrators.
Always run estimates of normal workloads during your testing scenarios, to gain accurate performance information and feedback.

Supporting Additional Subnets For the DHCP service to support additional subnets on your network, you must first determine if the routers used to connect adjoining subnets can support relaying of BOOTP and DHCP messages. If routers cannot be used for DHCP and BOOTP relay, you can set up either of the following for each subnet:

A computer running either Windows 2000 Server or Windows NT Server 4.0 configured to use the DHCP Relay Agent component. This computer simply forwards messages back and forth between clients on the local subnet and a remote DHCP server, using the IP address of the remote server. The DHCP Relay Agent service is available only on computers running Windows 2000 Server or Windows NT Server 4.0.
A computer running Windows 2000 Server configured as a DHCP server for the local subnet. This server computer must contain and manage scope and other address-configurable information for the local subnet it serves.

DHCP Traffic DHCP traffic does not use significant network bandwidth during normal periods of usage. Typical DHCP traffic does not exceed 1 percent of overall network traffic. However, there are two phases of DHCP client configuration that generate some network traffic load. These phases are IP address lease and IP address renewal.

When a client initializes TCP/IP for the first time (and is configured as a DHCP client), its first step is to acquire an IP address using DHCP. This process, as described earlier, results in a conversation between the DHCP client and server consisting of four packets, the first of which is the client computer broadcasting a DHCPDiscover packet in an attempt to locate a DHCP server.

As shown in the initial lease process earlier in this chapter, the entire process of acquiring an IP address lease through DHCP takes a total of four packets, each varying between 342 and 590 bytes in size. This process, on a clean network (when no other network traffic is using bandwidth), takes less than 1 second (about 300 milliseconds) on 10BaseT media. Results depend on media type in use.

DHCP conversations generally occur in the following instances:

When a DHCP client initializes for the first time (all four frames are sent).
When an automatic renewal occurs, which is done every one-half lease life (four days by default, or every 96 hours). This communication takes two packets (DHCPRequest and DHCPAck) and lasts approximately 200 milliseconds.
When a client is moved to a new subnet (DHCPRequest, DHCPNak, then the four frames).
When a DHCP client replaces its network adapter (all four frames are sent).
Whenever a client manually refreshes or releases its address by using the Ipconfig utility.

If you want to reduce the amount of traffic generated by DHCP, it is possible to adjust the lease duration for IP address leases. This is done by using DHCP Manager, and adjusting Lease Duration.