Qtech QSW-2900-24T-AC [111/209] Option 82 supporting

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Figure 3-1 Typical DHCP relay agent application

DHCP relay agents can transparently transmit broadcast packets on DHCP clients or servers to the DHCP

servers or clients in other network segments.

In the process of dynamic IP address assignment through the DHCP relay agent, the DHCP client and DHCP

server interoperate with each other in a similar way as they do without the DHCP relay agent. The following sections

only describe the forwarding process of the DHCP relay agent. For the interaction process of the packets, see

Obtaining IP Addresses Dynamically.

1) The DHCP client broadcasts the DHCP-DISCOVER packet.

2) After receiving the packets, the network device providing the DHCP relay agent function unicasts the

packet to the designated DHCP server based on the configuration.

3) The DHCP server assigns IP addresses and transmits the configuration information to the clients through

the DHCP relay agent so that the clients can be configured dynamically. The transmission mode depends on the flag

field in the DHCP-DISCOVER packet. For details, see section DHCP Packet Format.

6.7.3 Option 82 Supporting

a) Introduction to option 82 supporting

Option 82 is a relay agent information option in DHCP packets. When a request packet from a DHCP client

travels through a DHCP relay agent on its way to the DHCP server, the DHCP relay agent adds option 82 into the

request packet. Option 82 includes many sub-options, but the DHCP server supports only sub-option 1 and

sub-option 2 at present. Sub-option 1 defines agent circuit ID (that is, Circuit ID) and sub-option 2 defines remote

agent ID (that is, Remote ID).

Option 82 enables a DHCP server to track the address information of DHCP relay agents, through which and

other proper software, you can achieve the DHCP assignment limitation and accounting functions.

b) Primary terminologies

· Option: A length-variable field in DHCP packets, carrying information such as part of the lease

information and packet type. It includes at least one option and at most 255 options.

· Option 82: Also known as relay agent information option. This option is a part of the Option field in

DHCP packet. According to RFC3046, option 82 lies before option 255 and after the other options.

Option 82 includes at least one sub-option and at most 255 sub-options. Currently, the commonly used

sub-options in option 82 are sub-option 1 and sub-option 2.