Manual

195
User Guide
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Option 82
DHCP packets are classified into 8 types with the same format as the older BOOTP
packets. The difference between DHCP packets and BOOTP packets is the Option Field.
The Option Field of the DHCP packet is used to expand the functions of the packet. For
example, the DHCP Server can transmit control information and Network parameters
via the Option field. For more details on the available DHCP Options, please refer to
IEEE RFC 2132.
Option 82 records the location of the DHCP Client. Upon receiving the DHCP-REQUEST
packet, the Switch adds Option 82 to the packet and then transmits the packet to
DHCP Server. Administrators can then see the location of the DHCP Client via Option
82. DHCP Servers supporting Option 82 also can set the distribution policy of IP
Addresses and the other parameters according to the Option 82 setting, providing
more flexible IP Address distribution.
Option 82 can contain up to 255 sub-options. If Option 82 is defined, a sub-option
should be defined in the packet as well. The Switch supports two of the available sub-
options: Circuit ID and Remote ID. Since there is no standard for the content of Option
82, different manufacturers define the sub-options of Option 82 to fit their needs. On
the Switch, the sub-options are defined as follows: The Circuit ID is defined as the Port
Number that receives the DHCP Request packets and its VLAN ID. The Remote ID is
defined as the MAC Address of the DHCP Snooping device which receives the DHCP
Request packets from the DHCP Clients.
DHCP Cheat Attack
During the process of a DHCP Request there is no Authentication mechanism between
Server and Client. If there are multiple DHCP Servers in the Network, conflicting IP
Addresses and Security breaches can occur. Common causes of Rogue DHCP
Servers are: