6-5
Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Chapter 6 Configuring the System
Changing IP Information
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 the following
conditions:
• 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 autoloading of a configuration file from a network server.
Figure 6-1 shows the sequence of messages that are exchanged between the
DHCP client and the DHCP server.
Figure 6-1 DHCP Request for IP Information from a DHCP Server
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 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” section on page 6-6.
Switch A
DHCPACK (unicast)
DHCPREQUEST (broadcast)
DHCPOFFER (unicast)
DHCPDISCOVER (broadcast)
DHCP server
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