Dhcp Client Request Process - Cisco Catalyst 9500 Manual

Performing Device Setup Configuration

DHCP Client Request Process

During DHCP-based autoconfiguration, your device (DHCP client) is automatically configured at startup
with IP address information and a configuration file.
With DHCP-based autoconfiguration, no DHCP client-side configuration is needed on your device. However,
you need to configure the DHCP server for various lease options associated with IP addresses.
If you want to use 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.
Note
We recommend a redundant connection between a switch stack and the DHCP, DNS, and TFTP servers. This
is to help ensure that these servers remain accessible in case one of the connected stack members is removed
from the switch stack.
The DHCP server for your device can be on the same LAN or on a different LAN than the device. If the
DHCP server is running on a different LAN, you should configure a DHCP relay device between your device
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 device.
DHCP Client Request Process
When you boot up your device, the DHCP client is invoked and requests configuration information from a
DHCP server when the configuration file is not present on the device. If the configuration file is present and
the configuration includes the ip address dhcp interface configuration command on specific routed interfaces,
the DHCP client is invoked and requests the IP address information for those interfaces.
This is the sequence of messages that are exchanged between the DHCP client and the DHCP server.
Figure 4: DHCP Client and Server Message Exchange
The client, Device 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 device receives depends
on how you configure the DHCP server.
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.
System Management Configuration Guide, Cisco IOS XE Fuji 16.8.x (Catalyst 9500 Switches)
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