Cisco Catalyst 3550 Series User Manual

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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter19 Configuring DHCP Features
Configuring DHCP Features
Enabling the DHCP Relay Agent and Option 82
In Cisco IOS Release 12.1(19)EA1, the implementation for the Option 82 Subscriber Identification
changed from the previous release. For more information about configuring the relay agent and option
82 when using DHCP snooping, see the “Upgrading from a Previous Software Release” section on
page 19-4.
Beginning in privileged EXEC mode, follow these steps to enable the DHCP relay agent and option 82
on the switch.
To disable the DHCP server and relay agent, use the no service dhcp global configuration command.
To disable the insertion and removal of the option-82 field, use the no ip dhcp relay information option
global configuration command.
Validating the Relay Agent Information Option 82
By default, the switch verifies that the option-82 field in DHCP reply packet it receives from the DHCP
server is valid. If an invalid message is received, the switch drops it. If a valid message is received, the
switch removes the option-82 field and forwards the packet.
If you want to disable this feature, use the no ip dhcp relay information check global configuration
command. When disabled, the switch does not validate the option-82 field for validity, but still removes
the option from the packet and forwards it. (This feature is not available when DHCP snooping is
enabled on the switch.)
Note If the switch receives a packet that contains the option-82 field from a DHCP client and the information
checking feature is enabled, the switch drops the packet because it is invalid. However, in some
instances, you might configure a client with the option-82 field. In this situation, you should disable the
information-check feature so that the switch does not remove the option-82 field from the packet. You
can configure the action that the switch takes when it receives a packet with existing option-82
information by using the ip dhcp relay information policy global configuration command. For more
information, see the “Configuring the Reforwarding Policy” section on page 19-7. (This feature is not
available when DHCP snooping is enabled 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
ip dhcp relay information option Enable the switch to insert and remove DHCP relay information
(option-82 field) in forwarded DHCP request messages to the DHCP
server.
By default, this feature is disabled.
Step 4
end Return to privileged EXEC mode.
Step 5
show running-config Verify your entries.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.

Cisco Catalyst 3550 Series Specifications

General

Brand	Cisco
Model	Catalyst 3550 Series
Category	Network Router
Language	English