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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter5 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
With DHCP-based autoconfiguration, no DHCP client-side configuration is needed on your switch.
However, you need to configure the DHCP server for various lease options associated with IP addresses.
If you are using DHCP to relay the configuration file location on the network, you might also need to
configure a TFTP server and a Domain Name System (DNS) server.
The DHCP server for your switch can be on the same LAN or on a different LAN than the s witch. If the
DHCP server is running on a different LAN, you should configure a DHCP r ela y device be tween y our
switch and the DHCP server. A relay device forwards broadcast traffic between two directly connected
LANs. A router does not forward broadcast packets, but it forwards packets based on the destination IP
address in the received packet.
DHCP-based autoconfiguration replaces the BOOTP client functionality on your switch.
DHCP Client Request Process
When you boot your switch, the DHCP client is invoked and requests configuration information from a
DHCP server when the configuration file is not present on the switch.
DHCP autoconfiguration does not occur under these conditions:
•When a configuration file is present and the service config global configuration command is
disabled on the switch.
•When a configuration file is present and the service config global configuration command is enabled
on the switch. In this case, the switch broadcasts TFTP requests for the configuration file.
Figure 5-1 shows the sequence of messages that are exchanged between the DHCP client and the DHCP
server.
Figure5-1 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 fo rmal r eque st f or 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 t he 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 5-5.
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.
Switch A
DHCPACK (unicast)
DHCPREQUEST (broadcast)
DHCPOFFER (unicast)
DHCPDISCOVER (broadcast)
DHCP server
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