Cisco Catalyst 2960 Software Configuration Manual page 88

Chapter 3 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
Figure 3-1 shows the sequence of messages that are exchanged between the DHCP client and the DHCP
server.
Figure 3-1 DHCP Client and Server Message Exchange
DHCPDISCOVER (broadcast)
Switch A
DHCPOFFER (unicast)
DHCP server
DHCPREQUEST (broadcast)
DHCPACK (unicast)
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. For more information, see the "Configuring the TFTP
Server" section on page 3-7.
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.
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
3-4
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