Owner manual

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Москва, Новозаводская ул., 18, стр. 1

209

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.

29.2.1 Option 82 Working Mechanism

DHCP option 82 flow chart

If the DHCP Relay Agent 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 Relay Agent will add the option 82 to the end of the request message it receives,

then relay and forward the message to the DHCP server. 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), but the users can configure the Circuit ID as they wish. The

sub-option 2 of option 82(Remote ID) is the MAC address of the DHCP relay device.

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 Relay Agent.

4) DHCP Relay Agent will peel the option 82 information from the replay message sent by

DHCP

Server

DHCP Client

DHCP Relay Agent

DHCP

Request

DHCP

Request

Option8

DHCP