Cisco Catalyst 3560-X Series, Руководство по настройке программного обеспечения

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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
–
To ensure that the lease time in the database is accurate, we recommend that you enable and
configure NTP. For more information, see the
“Understanding Network Time Protocol” section
on page 1-2 .
– 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.
•
When you configure DHCP snooping smart logging, the contents of packets dropped by DHCP are
sent to a NetFlow collector. If you configure this feature, make sure that smart logging is globally
enabled. For more information about smart logging, see the
“Configuring Smart Logging” section
on page 1-14 .
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 12.4 .
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 1, “Managing Switch Stacks.”