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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
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
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.
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Version 3.0 - Version Date: September 20, 2005
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