ECS4100 Series CLI Reference Guide-R07

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 policy 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 packetExample
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:
■
sub-type - Distinguishes different types of circuit IDs.
Table 61: 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