Dhcp Client Request Process - Cisco Catalyst 4500 series Administration Manual

Chapter 3 Configuring the Switch for the First Time
Configuring DHCP-Based Autoconfiguration
With DHCP-based autoconfiguration, no DHCP client-side configuration is needed on your switch
because your switch (the DHCP client) is automatically configured at startup with IP address
information and a configuration file. However, you need to configure the DHCP server or the DHCP
server feature on your switch 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 Trivial
File Transfer Protocol (TFTP) server and a Domain Name System (DNS) server.
DHCP-based autoconfiguration replaces the BOOTP client functionality on your switch.

DHCP Client Request Process

At startup the switch automatically requests configuration information from a DHCP server if a
configuration file is not present on the switch.
Figure 3-1 shows the sequence of messages that are exchanged between the DHCP client and the DHCP
server.
Figure 3-1 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, 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 the
configuration information that it 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 3-4.
If the configuration parameters sent to the client in the DHCPOFFER unicast message are invalid (if
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 might have assigned the parameters to another client.)
A DHCP client might receive offers from multiple DHCP servers and can accept any of them; 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.
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
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