Chapter 9
| General Security Measures
DHCPv4 Snooping
– 309 –
Command Mode
Global Configuration
Command Usage
â—† 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.
â—† 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.
â—† 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.
â—† 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:
â–
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 option tr101 board-id command.
â–
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.
â–
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 packet.
Example
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
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