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Chapter 21
Configuring DHCP Features and IP Source Guard
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
Configuring the DHCP Relay Agent
Beginning in privileged EXEC mode, follow these steps to enable the DHCP relay agent on the switch:
Command
Step 1
configure terminal
Step 2
service dhcp
Step 3
end
Step 4
show running-config
Step 5
copy running-config startup-config
To disable the DHCP server and relay agent, use the no service dhcp global configuration command.
Refer to the "Configuring DHCP" section of the "IP Addressing and Services" section of the Cisco IOS
IP Configuration Guide, Release 12.2 for these procedures:
•
•
Specifying the Packet Forwarding Address
If the DHCP server and the DHCP clients are on different networks or subnets, you must configure the
switch with the ip helper-address address interface configuration command. The general rule is to
configure the command on the Layer 3 interface closest to the client. The address used in the ip
helper-address command can be a specific DHCP server IP address, or it can be the network address if
other DHCP servers are on the destination network segment. Using the network address enables any
DHCP server to respond to requests.
78-16180-02
Purpose
Enter global configuration mode.
Enable the DHCP server and relay agent on your switch. By default, this
feature is enabled.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Checking (validating) the relay agent information
Configuring the relay agent forwarding policy
Chapter 5, "Managing Switch Stacks."
Catalyst 3750 Switch Software Configuration Guide
Configuring DHCP Features
21-9
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