When you boot up 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.
This is the sequence
of messages that are exchanged between the DHCP client and the DHCP server.
Figure 1. DHCP Client and
Server Message Exchange
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.
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
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 is 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.
The DHCP hostname
option allows a group of
switches to obtain hostnames and a standard
configuration from the central management DHCP server. A client (switch) includes in its DCHPDISCOVER message
an option 12 field used to request a hostname and other configuration
parameters from the DHCP server. The configuration files on all clients are
identical except for their DHCP-obtained hostnames.
If a client has a
default hostname (the
name global configuration command is not
configured or the
global configuration command is entered to remove the hostname), the DHCP
hostname option is not included in the packet when you enter the
ip address dhcp
interface configuration command. In this case, if the client receives the DCHP
hostname option from the DHCP interaction while acquiring an IP address for an
interface, the client accepts the DHCP hostname option and sets the flag to
show that the system now has a hostname configured.