The client, Switch 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 switch 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 switch accepts replies from a BOOTP server and
configures itself, the switch broadcasts, instead of unicasts, TFTP requests to obtain the switch configuration
file.
The DHCP hostname option allows a group of switches to obtain hostnames and a standard configuration
from the central management DHCP server. A client (switch) 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.
If a client has a default hostname (the hostname name global configuration command is not configured or
the no hostname global configuration command is entered to remove the hostname), the DHCP hostname
option is not included in the packet when you enter the ip address dhcp interface configuration command.
In this case, if the client receives the DCHP hostname option from the DHCP interaction while acquiring an
IP address for an interface, the client accepts the DHCP hostname option and sets the flag to show that the
system now has a hostname configured.
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 switches in a network. Simultaneous image and configuration
upgrade for all switches in the network helps ensure that each new switch added to a network receives the
same image and configuration.
There are two types of DHCP image upgrades: DHCP autoconfiguration and DHCP auto-image update.
Restrictions for DHCP-based Autoconfiguration
•
The DHCP-based autoconfiguration with a saved configuration process stops if there is not at least one
Layer 3 interface in an up state without an assigned IP address in the network.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
1562
Information About Performing Switch Setup Configuration
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