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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring DHCP Features and IP Source Guard
Configuring DHCP Features
DHCP Snooping Configuration Guidelines
• 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.
• 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.
• When configuring a large number of circuit IDs on a switch, consider the impact of lengthy character
strings on the NVRAM or the flash memory. If the circuit-ID configurations, combined with other
data, exceed the capacity of the NVRAM or the flash memory, an error message appears.
• 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.
–
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.
Cisco IOS DHCP server binding database Enabled in Cisco IOS software, requires
configuration.
Note The switch gets network addresses and
configuration parameters only from a
device configured as a DHCP server.
DHCP snooping binding database agent Enabled in Cisco IOS software, requires
configuration. This feature is operational only
when a destination is configured.
1. The switch responds to DHCP requests only if it is configured as a DHCP server.
2. The switch relays DHCP packets only if the IP address of the DHCP server is configured on the SVI of the DHCP client.
3. Use this feature when the switch is an aggregation switch that receives packets with option-82 information from an edge
switch.
Table 1-1 Default DHCP Configuration (continued)
Feature Default Setting
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