Cisco IE 4000, Software Configuration Manual

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Performing Switch Setup Configuration
Information About Performing Switch Setup Configuration
Figure 2 DHCP Client and Server Message Exchange
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 serverd in
conjunction with the TFTP server. For more information, see
TFTP Server, page 65 .
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 switch. 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. This 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.
Switch A
DHCPACK (unicast)
DHCPREQUEST (broadcast)
DHCPOFFER (unicast)
DHCPDISCOVER (broadcast)
DHCP server
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