23-8
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 23 Configuring DHCP Features and IP Source Guard
Configuring DHCP Snooping
•
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 8-3
.
–
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.
•
You can display DHCP snooping statistics by entering the
show ip dhcp snooping statistics
user
EXEC command, and you can clear the snooping statistics counters by entering the
clear ip dhcp
snooping statistics
privileged EXEC command.
Note
Do not enable Dynamic Host Configuration Protocol (DHCP) snooping on RSPAN VLANs. If
DHCP snooping is enabled on RSPAN VLANs, DHCP packets might not reach the RSPAN
destination port.
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.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
service dhcp
Enable the DHCP server and relay agent on your switch. By default, this
feature is enabled.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entries.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Содержание IE 3000
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