CLI Reference Guide-R02

Chapter 9 | General Security Measures
DHCPv4 Snooping
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ip dhcp snooping
information option
This command enables the use of DHCP Option 82 information for the switch, and
specifies the frame format to use for the remote-id when Option 82 information is
generated by the switch. Use the no form without any keywords to disable this
function.
Syntax
ip dhcp snooping information option
no ip dhcp snooping information option
Default Setting
Disabled
Command Mode
Global Configuration
Command Usage
◆ DHCP provides a relay mechanism for sending information about the switch
and its DHCP clients to the DHCP server. Known as DHCP Option 82, it allows
compatible DHCP servers to use the information when assigning IP addresses,
or to set other services or policies for clients.
◆ When the DHCP Snooping Information Option 82 is enabled, the requesting
client (or an intermediate relay agent that has used the information fields to
describe itself) can be identified in the DHCP request packets forwarded by the
switch and in reply packets sent back from the DHCP server.
◆ When the DHCP Snooping Information Option is enabled, clients can be
identified by the switch port to which they are connected rather than just their
MAC address. DHCP client-server exchange messages are then forwarded
directly between the server and client without having to flood them to the
entire VLAN.
◆ 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