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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Configuration and Operation

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DHCP Early Lease Termination (Release) Process
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DHCP Parameter Configuration Process For Clients With Non-DHCP Addresses
(Page 2 of 2)

Parameter Configuration Process Steps

The following steps show how a device with an externally-configured address uses DHCP to get other parameters (see Figure 266 as well).

Figure 266: DHCP Parameter Configuration Process

A device that already has an IP address can use the simple request/reply exchange shown above to get other configuration parameters from a DHCP server. In this case I have shown the client broadcasting its request.


1. Client Creates DHCPINFORM Message

The client (which again, may be a DHCP server acting as a client) creates a DHCPINFORM message. It fills in its own IP address in the CIAddr field, since that IP address is current and valid. It may request specific parameters using the Parameter Request List option, or simply accept the defaults provided by the server.

2. Client Sends DHCPINFORM Message

The client sends the DHCPINFORM message unicast, if it knows the identity and address of a DHCP server, otherwise, it broadcasts it.

3. Server Receives and Processes DHCPINFORM Message

The message is received and processed by the DHCP server or servers (if there are multiple and the request was broadcast). Each server checks to see if it has the parameters needed by the client in its database.

4. Server Creates DHCPACK Message

Each server that has the information the client needs creates a DHCPACK message, which includes the needed parameters in the appropriate DHCP option fields. (Often this will be only a single server.)

5. Server Sends DHCPACK Message

The server sends the message unicast back to the client.

6. Client Receives and Processes DHCPACK Message

The client receives the DHCPACK message sent by the server, processes it, and sets its parameters accordingly.

Dealing with Failure of the Configuration Process

If a client receives no reply to its DHCPINFORM message it will retransmit it periodically. After a retry period it will give up and use default configuration values. It will also typically generate an error report to inform an administrator or user of the problem.

Key Concept: Devices that are not using DHCP to acquire IP addresses can still utilize its other configuration capabilities. A client can broadcast a DHCPINFORM message to request that any available server send it parameters for how the network is to be used. DHCP servers respond with the requested parameters and/or default parameters, carried in DHCP options of a DHCPACK message.


Previous Topic/Section
DHCP Early Lease Termination (Release) Process
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DHCP Messaging, Message Types and Formats
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Version 3.0 - Version Date: September 20, 2005

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