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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 3 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
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 TFTP
Server” section on page 3-7
.
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 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 hostname name global configuration command is not configured
or the no hostname 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.
Understanding DHCP-based Autoconfiguration and Image Update
You can use the DHCP image upgrade features to configure a DHCP server to download both a new
image and a new configuration file to one or more switches in a network. This helps ensure that each
new switch added to a network receives the same image and configuration.
There are two types of DHCP image upgrades: DHCP autoconfiguration and DHCP auto-image update.
DHCP Autoconfiguration
DHCP autoconfiguration downloads a configuration file to one or more switches in your network from
a DHCP server. The downloaded configuration file becomes the running configuration of the switch. It
does not over write the bootup configuration saved in the flash, until you reload the switch.
DHCP Auto-Image Update
You can use DHCP auto-image upgrade with DHCP autoconfiguration to download both a configuration
and a new image to one or more switches in your network. The switch (or switches) downloading the
new configuration and the new image can be blank (or only have a default factory configuration loaded).
