D-Link DES-3526 Manual page 131

DHCP Agent
Information Option 82
Check
DHCP Agent
Information Option 82
Policy
Click Apply to implement any changes that have been made.
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 configuring the DHCP
Agent Information Option 82 Policy.
The Implementation of DHCP Information Option 82 in DES-3526
The config dhcp_relay option_82 command configures the DHCP relay agent information option 82
setting of the switch. The formats for the circuit ID sub-option and the remote ID sub-option are as
follows:
DES-3526 / DES-3526DC Fast Ethernet Layer 2 Switch
the packet. Once the option 82 information has been added to the packet it is sent on
to the DHCP server. When the DHCP server receives the packet, if the server is
capable of option 82, it can implement policies like restricting the number of IP
addresses that can be assigned to a single remote ID or circuit ID. Then the DHCP
server echoes the option 82 field in the DHCP reply. The DHCP server unicasts the
reply to the back to the relay agent if the request was relayed to the server by the
relay agent. The switch verifies that it originally inserted the option 82 data. Finally,
the relay agent removes the option 82 field and forwards the packet to the switch port
that connects to the DHCP client that sent the DHCP request.
Disabled- If the field is toggled to Disabled the relay agent will not insert and remove
DHCP relay information (option 82 field) in messages between DHCP servers and
clients, and the check and policy settings will have no effect.
This field can be toggled between Enabled and Disabled using the pull-down menu.
It is used to enable or disable the Switches ability to check the validity of the packet's
option 82 field.
Enabled– When the field is toggled to Enable, the relay agent will check the validity of
the packet's option 82 field. If the switch receives a packet that contains the option-82
field from a DHCP client, the switch drops the packet because it is invalid. In packets
received from DHCP servers, the relay agent will drop invalid messages.
Disabled- When the field is toggled to Disabled, the relay agent will not check the
validity of the packet's option 82 field.
This field can be toggled between Replace, Drop, and Keep by using the pull-down
menu. It is used to set the Switches policy for handling packets when the DHCP
Agent Information Option 82 Check is set to Disabled. The default is Replace.
Replace- The option 82 field will be replaced if the option 82 field already exists in the
packet received from the DHCP client.
Drop- The packet will be dropped if the option 82 field already exists in the packet
received from the DHCP client.
Keep-The option 82 field will be retained if the option 82 field already exists in the
packet received from the DHCP client.
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