Dhcp Client Request Process; Dhcp-Based Autoconfiguration And Image Update - Cisco Catalyst 9200 Configuration Manual
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Performing Device Setup Configuration
DHCP Client Request Process
When you boot up your device, the DHCP client is invoked and requests configuration information from a
DHCP server when the configuration file is not present on the device. If the configuration file is present and
the configuration includes the ip address dhcp interface configuration command on specific routed interfaces,
the DHCP client is invoked and requests the IP address information for those interfaces.
This is the sequence of messages that are exchanged between the DHCP client and the DHCP server.
Figure 3: DHCP Client and Server Message Exchange
The client, Device A, broadcasts a DHCPDISCOVER message to locate a DHCP server. The DHCP server
offers configuration parameters (such as an IP address, subnet mask, gateway IP address, DNS IP address, a
lease for the IP address, and so forth) to the client in a DHCPOFFER unicast message.
In a DHCPREQUEST broadcast message, the client returns a formal request for the offered configuration
information to the DHCP server. The formal request is broadcast so that all other DHCP servers that received
the DHCPDISCOVER broadcast message from the client can reclaim the IP addresses that they offered to
the client.
The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK
unicast message to the client. With this message, the client and server are bound, and the client uses
configuration information received from the server. The amount of information the device receives depends
on how you configure the DHCP server.
If the configuration parameters sent to the client in the DHCPOFFER unicast message are invalid (a
configuration error exists), the client returns a DHCPDECLINE broadcast message to the DHCP server.
The DHCP server sends the client a DHCPNAK denial broadcast message, which means that the offered
configuration parameters have not been assigned, that an error has occurred during the negotiation of the
parameters, or that the client has been slow in responding to the DHCPOFFER message (the DHCP server
assigned the parameters to another client).
A DHCP client might receive offers from multiple DHCP or BOOTP servers and can accept any of the offers;
however, the client usually accepts the first offer it receives. The offer from the DHCP server is not a guarantee
that the IP address is allocated to the client; however, the server usually reserves the address until the client
has had a chance to formally request the address. If the device accepts replies from a BOOTP server and
configures itself, the device broadcasts, instead of unicasts, TFTP requests to obtain the device configuration
file.
The DHCP hostname option allows a group of devices to obtain hostnames and a standard configuration from
the central management DHCP server. A client (device) includes in its DCHPDISCOVER message an option
12 field used to request a hostname and other configuration parameters from the DHCP server. The configuration
files on all clients are identical except for their DHCP-obtained hostnames.
DHCP-based Autoconfiguration and Image Update
You can use the DHCP image upgrade features to configure a DHCP server to download both a new image
and a new configuration file to one or more devices in a network. Simultaneous image and configuration
System Management Configuration Guide, Cisco IOS XE Gibraltar 16.10.x (Catalyst 9200 Switches)
DHCP Client Request Process
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