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Catalyst 2960 Switch SoftwareConfiguration Guide
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Chapter19 Configuring DHCP Features
Configuring DHCP Features
•Before configuring the DHCP snooping information option on your switch, be sure to configu re the
device that is acting as the DHCP server. For example, you must specify the IP addresses that the
DHCP server can assign or exclude, or you must configure DHCP options for these devices.
•If the DHCP relay agent is enabled but DHCP snooping is disabled, the DHCP option-82 data
insertion feature is not supported.
•If a switch port is connected to a DHCP server, configure a port as trusted by entering the ip dhcp
snooping trust interface configuration command.
•If a switch port is connected to a DHCP client, configure a port as untrusted by entering the no ip
dhcp snooping trust interface configuration command.
•Follow these guidelines when configuring the DHCP snooping binding database:
–
Because both NVRAM and the flash memory have limited storage capacity, we recommend that
you store the binding file on a TFTP server.
–
For network-based URLs (such as TFTP and FTP), you must create an empty file at the
configured URL before the switch can write bindings to the binding file at that URL. See the
documentation for your TFTP server to determine whether you must first create an empty file
on the server; some TFTP servers cannot be configured this way.
–
To ensure that the lease time in the database is accurate, we recommend that you enable and
configure NTP. For more information, see the “Configuring NTP” section on page 6-4.
–
If NTP is configured, the switch writes binding changes to the binding file only when the switch
system clock is synchronized with NTP.
•Do not enter the ip dhcp snooping information option allow-untrusted command on an
aggregation switch to which an untrusted device is connected. If you enter this command, an
untrusted device might spoof the option-82 information.
Configuring the DHCP Relay Agent
Beginning in privileged EXEC mode, follow these steps to enable the DHCP relay agent on the switch:
To disable the DHCP server and relay agent, use the no service dhcp global configuration command.
See the “Configuring DHCP” section of the “IP Addressing and Services” section of the Cisco IOS IP
Configuration Guide, Release12.2 for these procedure s:
•Checking (validating) the relay agent information
•Configuring the relay agent forwarding policy
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 service dhcp Enable the DHCP server and relay agent on your switch. By default, this
feature is enabled.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entries.
Step5 copy running-config startup-config (Optional) Save your entries in the configuration file.