Upgrading From A Previous Software Release - Cisco Catalyst 3550 series Software Configuration Manual

Configuring DHCP Features
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Upgrading from a Previous Software Release

In Cisco IOS Release 12.1(19)EA1, the implementation for the Option 82 Subscriber Identification
changed from the previous release. The new option-82 format uses a different circuit ID and remote ID
suboption, vlan-mod-port. The previous version uses the snmp-ifindex circuit ID and remote ID
suboption.
If you have option 82 configured on the switch and you upgrade to Cisco IOS Release 12.1(19)EA1 or
later, the option-82 configuration is not affected. However, when you globally enable DHCP snooping
on the switch by using the ip dhcp snooping global configuration command, the previous option-82
configuration is suspended, and the new option-82 format is applied. When you globally disable DHCP
snooping on the switch, the previous option-82 configuration is re-enabled.
To provide for backward compatibility, you can select the previous option-82 format by using the ip
dhcp snooping information option format snmp-ifindex global configuration command when you
enable DHCP snooping. When DHCP snooping is globally enabled, option-82 information (in the
selected format) is only inserted on snooped VLANs.
To use the previous version of option 82 without enabling DHCP snooping, see the
Relay Agent and Option 82" section on page 19-6
Enabling DHCP Snooping and Option 82
Beginning in privileged EXEC mode, follow these steps to enable DHCP snooping on the switch.
Command
Step 1 configure terminal
Step 2
ip dhcp snooping
Catalyst 3550 Multilayer Switch Software Configuration Guide
19-4
When you globally enable DHCP snooping on the switch, these Cisco IOS commands are not
available until snooping is disabled. If you enter these commands, the switch returns an error
message, and the configuration is not applied.
ip dhcp relay information check global configuration command
–
– ip dhcp relay information policy global configuration command
– ip dhcp relay information trust-all global configuration command
– ip dhcp relay information option global configuration command
– ip dhcp relay information trusted interface configuration command
Before configuring the DHCP information option on your switch, make sure to configure the device
that is acting as the DHCP server. For example, you must specify the IP addresses that the DHCP
server can assign or exclude or configure DHCP options for devices.
– If your switch is the DHCP server, see the
for more information.
– If your DHCP server is a Cisco device, refer to the "IP Addressing and Services" section in the
"Configuring DHCP" chapter of the Cisco IOS IP and IP Routing Configuration Guide for
Release 12.1. Otherwise, refer to the documentation that shipped with the server.
Purpose
Enter global configuration mode.
Enable DHCP snooping globally.
Chapter 19
"Configuring the DHCP Server" section on page 4-5
for instructions.
Configuring DHCP Features
"Enabling the DHCP
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