Cisco Catalyst 3750-E Series, Software Configuration Manual

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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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Chapter 22 Configuring DHCP Features and IP Source Guard
Configuring DHCP Features
–
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 7-4 .
– 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.
• Starting with Cisco IOS Release 12.2(37)SE, 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 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, see the “Configuring DHCP” section of the
“IP addressing and Services” section of the
Cisco IOS IP Configuration Guide, Release 12.2 .
DHCP Server and Switch Stacks
The DHCP binding database is managed on the stack master. When a new stack master is assigned, the
new master downloads the saved binding database from the TFTP server. If the stack master fails, all
unsaved bindings are lost. The IP addresses associated with the lost bindings are released. You should
configure an automatic backup by using the
ip dhcp database url [ timeout seconds | write-delay
seconds ] global configuration command.
When a stack merge occurs, the stack master that becomes a stack member loses all of the DHCP lease
bindings. With a stack partition, the new master in the partition acts as a new DHCP server without any
of the existing DHCP lease bindings.
For more information about the switch stack, see Chapter 5, “Managing Switch Stacks.”
Configuring the DHCP Relay Agent
Beginning in privileged EXEC mode, follow these steps to enable the DHCP relay agent on the switch:
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.