Dhcp Client Request Process - Cisco WS-C3550-12G Software Configuration Manual

Chapter 4 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information

DHCP Client Request Process

When you boot your switch, the DHCP client is invoked and automatically requests configuration
information from a DHCP server when the configuration file is not present on the switch.
Figure 4-1 shows the sequence of messages that are exchanged between the DHCP client and the DHCP
server.
Figure 4-1 DHCP Client and Server Message Exchange
DHCPDISCOVER (broadcast)
Switch A
DHCPOFFER (unicast)
DHCP server
DHCPREQUEST (broadcast)
DHCPACK (unicast)
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 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 "Configuring the DHCP
Server" 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 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 broadcasts, instead of unicasts, TFTP requests to
obtain the switch configuration file.
Catalyst 3550 Multilayer Switch Software Configuration Guide
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