CLI Reference Guide-R04

Chapter 9 | General Security Measures
DHCPv4 Snooping
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◆ DHCP snooping must be enabled for the DHCP Option 82 information to be
inserted into packets. When enabled, the switch will only add/remove option
82 information in incoming DHCP packets but not relay them. Packets are
processed as follows:
■
If an incoming packet is a DHCP request packet with option 82 information,
it will modify the option 82 information according to settings specified with
ip dhcp snooping information option tr101 board-id command.
■
If an incoming packet is a DHCP request packet without option 82
information, enabling the DHCP snooping information option will add
option 82 information to the packet.
■
If an incoming packet is a DHCP reply packet with option 82 information,
enabling the DHCP snooping information option will remove option 82
information from the packet.
Example
This example enables the DHCP Snooping Information Option.
Console(config)#ip dhcp snooping information option
Console(config)#
ip dhcp snooping
information option
encode no-subtype
This command disables the use of sub-type and sub-length fields for the
circuit-ID (CID) and remote-ID (RID) in Option 82 information generated by the
switch. Use the no form to enable the use of these fields.
Syntax
[no] ip dhcp snooping information option encode no-subtype
Default Setting
CID/RID sub-type: Enabled
Command Mode
Global Configuration
Command Usage
◆ Option 82 information generated by the switch is based on TR-101 syntax as
shown below:
The circuit identifier used by this switch starts at sub-option1 and goes to the
end of the R-124 string. The R-124 string includes the following information:
Table 59: Option 82 information
82 3-69 1 1-67 x1 x2 x3 x4 x5 x63
opt82 opt-len sub-opt1 string-len R-124 string