Understanding Dhcp-Based Autoconfiguration; Dhcp Client Request Process - Cisco 3845 - Security Bundle Router Software Manual

Assigning Switch Information

Understanding DHCP-Based Autoconfiguration

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 a mechanism 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. The switch can act as both a DHCP client and a DHCP server.
During DHCP-based autoconfiguration, your switch (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 switch.
However, you need to configure the DHCP server 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.
The DHCP server for your switch can be on the same LAN or on a different LAN than the switch. If the
DHCP server is running on a different LAN, you should configure a DHCP relay device between your
switch 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 switch.

DHCP Client Request Process

When you boot your switch, the DHCP client is invoked and requests configuration information from a
DHCP server when the configuration file is not present on the switch. 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.
Figure 3-1
server.
Figure 3-1
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.
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.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
3-4
shows the sequence of messages that are exchanged between the DHCP client and the DHCP
DHCP Client and Server Message Exchange
DHCPDISCOVER (broadcast)
DHCPOFFER (unicast)
DHCPREQUEST (broadcast)
DHCPACK (unicast)
Chapter 3 Assigning the Switch IP Address and Default Gateway
DHCP server
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