QTech QSW-2800 series, Configuration Manual

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Москва, Новозаводская ул., 18, стр. 1
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SubOpt: the sequence number of sub-option, the sequence number of Circuit ID sub-option is
1, the sequence number of Remote ID sub-option is 2.
Len: the number of bytes in Sub-option Value, not including the two bytes in SubOpt segment
and Len segment.
33.1.2 DHCP Snooping option 82 Working Mechanism
DHCP option 82 flow chart
If the DHCP SNOOPING supports option 82, the DHCP client should go through the following
four steps to get its IP address from the DHCP server: discover, offer, select and acknowledge.
The DHCP protocol follows the procedure below:
1) DHCP client sends a request broadcast message while initializing. This request message
does not have option 82.
2) DHCP SNOOPING will add the option 82 to the end of the request message it receives, and
perform layer 2 forwarding. By default, the sub-option 1 of option 82 (Circuit ID) is the interface
information of the switch connected to the DHCP client (VLAN name and physical port name).
The sub-option 2 of option 82(Remote ID) is the CPU MAC address of the switch.
3) After receiving the DHCP request message, the DHCP server will allocate IP address and
other information for the client according to the information and preconfigured policy in the
option segment of the message. Then it will forward the reply message with DHCP
configuration information and option 82 information to DHCP SNOOPING.
4) DHCP SNOOPING will peel the option 82 information from the replay message sent by
DHCP server, then the message with DHCP configuration information to perform layer 2
DHCP
Server
DHCP Client
DHCP SNOOPING
DHCP
Request
DHCP
Request
Option8
2
Option8
2
DHCP
Reply
DHCP
Reply