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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

DHCP Early Lease Termination (Release) Process 1 2 DHCP Messaging, Message Types and Formats

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).

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.

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

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.

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.)

The server sends the message unicast back to the client.

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

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.

DHCP Early Lease Termination (Release) Process 1 2 DHCP Messaging, Message Types and Formats