Korenix JetNet 5228G Series, User Manual

Page: 461 / 658

Client Security Commands
4-151
4
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
This example enables the DHCP Snooping Information Option.
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.
Console(config)#ip dhcp snooping information option
Console(config)#