Understanding Dhcp-Based Autoconfiguration; Dhcp Client Request Process - Cisco Catalyst 2950 Software Manual

Chapter 6 Configuring the System
DHCP replaces the Bootstrap Protocol (BOOTP) feature autoconfiguration to ensure retrieval of
Note
configuration files by unicast TFTP messages. BOOTP is available in earlier software releases for this
switch.

Understanding DHCP-Based Autoconfiguration

The DHCP provides configuration information to Internet hosts and internetworking devices. This
protocol consists of two components: one for delivering configuration parameters from a DHCP server
to a device and one for allocating network addresses to devices. DHCP is built on a client-server model,
in which designated DHCP servers allocate network addresses and deliver configuration parameters to
dynamically configured devices.
With DHCP-based autoconfiguration, your switch (DHCP client) can be automatically configured at
startup with IP address information and a configuration file that it receives during DHCP-based
autoconfiguration. No DHCP client-side configuration is required on your switch.
However, you need to configure the DHCP server for various lease options. You might also need to
configure a TFTP server, a Domain Name System (DNS) server, and possibly a relay device if the servers
are on a different LAN than your switch. 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 can be invoked and automatically request configuration
information from a DHCP server under these conditions:
•
•
•
Figure 6-1
server.
Figure 6-1
Switch A
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.
78-11380-03
The configuration file is not present on the switch.
The configuration file is present, but the IP address is not specified in it.
The configuration file is present, the IP address is not specified in it, and the service config global
configuration command is included. This command enables the auto-loading of a configuration file
from a network server.
shows the sequence of messages that are exchanged between the DHCP client and the DHCP
DHCP Request for IP Information from a DHCP Server
DHCPDISCOVER (broadcast)
DHCPOFFER (unicast)
DHCPREQUEST (broadcast)
DHCPACK (unicast)
DHCP server
Catalyst 2950 Desktop Switch Software Configuration Guide
Changing IP Information
6-3