6-4
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter 6 Configuring the System
Changing IP Information
In a DHCPREQUEST broadcast message, the client returns a request for the offered configuration
information to the DHCP server. The 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 DHCP
Server” sectiononpage6-4.
If the configuration parameters sent to the client in the DHCPOFFER unicast message by the DHCP
server 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) of the DHCP server.
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 will be 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 will broadcast, instead of unicast, TFTP requests
to obtain the switch configuration file.
Configuring the DHCP Server
You should configure the DHCP servers with reserved leases that are bound to each switch by the switch
hardware address. If the DHCP server does not support reserved leases, the switch can obtain different
IP addresses and configuration files at different boot instances. You should configure the DHCP server
with these lease options:
• IP address of the client (required)
• Subnet mask of the client (required)
• DNS server IP address (required)
• Router IP address (default gateway address to be used by the switch) (required)
• TFTP server name (required)
• Boot filename (the name of the configuration file that the client needs) (recommended)
• Host name (optional)
If you do not configure the DHCP server with the lease options described earlier, it replies to client
requests with only those parameters that have available values. If the IP address and subnet mask are not
in the reply, the switch is not configured. If the DNS server IP address, router IP address, or TFTP server
name are not found, the switch might broadcast TFTP requests. Unavailability of other lease options
does not affect autoconfiguration.
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