Dhcp Snooping Configuration Guidelines; Configuring The Dhcp Server - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring DHCP Features

DHCP Snooping Configuration Guidelines

These are the configuration guidelines for DHCP snooping.
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Configuring the DHCP Server

The switch can act as a DHCP server. By default, the Cisco IOS DHCP server and relay agent features
are enabled on your switch but are not configured. These features are not operational.
For procedures to configure the switch as a DHCP server, refer to the "Configuring DHCP" section of
the "IP addressing and Services" section of the Cisco IOS IP Configuration Guide, Release 12.2.
Catalyst 3750 Switch Software Configuration Guide
21-8
You must globally enable DHCP snooping on the switch.
DHCP snooping is not active until DHCP snooping is enabled on a VLAN.
Before globally enabling DHCP snooping on the switch, make sure that the devices acting as the
DHCP server and the DHCP relay agent are configured and enabled.
When you globally enable DHCP snooping on the switch, these Cisco IOS commands are not
available until snooping is disabled. If you enter these commands, the switch returns an error
message, and the configuration is not applied.
ip dhcp relay information check global configuration command
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– ip dhcp relay information policy global configuration command
ip dhcp relay information trust-all global configuration command
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ip dhcp relay information trusted interface configuration command
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Before configuring the DHCP snooping information option on your switch, be sure to configure 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.
Before configuring the DHCP relay agent on your switch, make sure to configure 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, configure DHCP options for devices, or set up the DHCP database agent.
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.
– You must create an empty file at the configured URL on network-based URLs (such as TFTP
and FTP) before the switch can initially write bindings to the binding file at that URL for the
first time.
– To ensure that the lease time in the database is accurate, we recommend that NTP is enabled and
configured. For more information, see the
If NTP is not configured, the switch writes binding changes to the binding file only when the
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switch system clock is synchronized with NTP.
Chapter 21 Configuring DHCP Features and IP Source Guard
"Configuring NTP" section on page 7-4.
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