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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 4 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
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 “DHCP Server
Configuration Guidelines” section on page 4-5.
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.
Configuring DHCP-Based Autoconfiguration
These sections describe how to configure DHCP-based autoconfiguration.
• DHCP Server Configuration Guidelines, page 4-5
• Configuring the TFTP Server, page 4-6
• Configuring the DNS, page 4-7
• Configuring the Relay Device, page 4-7
• Obtaining Configuration Files, page 4-8
• Example Configuration, page 4-9
If your DHCP server is a Cisco device, or if you are configuring the switch as a DHCP server, see the
“IP Addressing and Services” section in the Cisco IOS IP and IP Routing Configuration Guide for Cisco
IOS Release 12.1 for additional information about configuring DHCP.

DHCP Server Configuration Guidelines

Follow these guidelines if you are configuring a device as a DHCP server:
The switch can act as both the DHCP client and the DHCP server. By default, the Cisco IOS DHCP
server and relay agent features are enabled on your switch.
Note The DHCP server feature is only available on Catalyst 2955 switches.
You should configure the DHCP server with reserved leases that are bound to each switch by the switch
hardware address.