Planet IGSW-2840 User Manual page 485

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 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 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 on the switch for the DHCP Option 82 information to be inserted into packets.
DHCP request packets are flooded onto all attached VLANs other than the inbound VLAN under the following situations:
-DHCP snooping is disabled. -The request packet contains a valid relay agent address field.
• DHCP reply packets are flooded onto all attached VLANs other than the inbound management VLAN under the following
situations: -The reply packet does not contain Option 82 information. -The reply packet contains a valid relay agent
address field (that is not the
address of this switch) or a zero relay address.
Use the ip dhcp snooping information option command (page 4-150) to specify how to handle DHCP client request
packets which already contain Option 82 information.
Example
Example
This enables the DHCP Snooping Information Option.
Console(config)#ip dhcp snooping information option
Console(config)#
ip dhcp snooping information policy
This command sets the DHCP snooping information option policy for DHCP client packets that include Option 82 information.
Syntax
ip dhcp snooping information policy {drop | keep | replace}
drop - Drop the request packet instead of relaying it.
keep - Retain the Option 82 information in the client request, and unicast the packet to the DHCP server.
replace -Replace the Option 82 information in the client's request with information about the relay agent itself, insert the
relay agent's address (when DHCP snooping is enabled), and unicast the packet to the DHCP server.
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User's Manual of IGSW-2840