96
The switch forwards the DHCP request that includes the option-82 field to the DHCP server.
The DHCP server receives the packet. If the server is option-82 capable, it can use the
remote ID, the circuit ID, or both to assign IP addresses and implement policies, such as
restricting the number of IP addresses that can be assigned to a single remote ID or circuit
ID. Then the DHCP server
echoes
the option-82 field in the DHCP reply.
The DHCP server unicasts the reply to the switch if the request was relayed to the server by
the switch. When the client and server are on the same subnet, the server broadcasts the
reply. The switch verifies that it originally inserted the option-82 data by inspecting the
remote ID and possibly the circuit ID fields. The switch
removes
the option-82 field and
forwards the packet to the switch port that connects to the DHCP client that sent the DHCP
request
.
Option Frame Format:
Code
Len
Agent Information Field
82
N
i1
i2
i3
i4
. . .
iN
The Agent Information field consists of a sequence of SubOpt/Length/Value tuples for each
sub-option, encoded in the following manner:
Sub-Option
Len
Sub-Option Value
1
N
s1
s2
s3
s4
. . .
sN
DHCP Agent Sub-Option Description
Sub-option Code
---------------
----------------------
1 Agent Circuit ID Sub-option
2 Agent Remote ID Sub-option
Circuit ID Sub-option Format:
Suboption
Type
Length
Information
0x01
Circuit Form
Remote ID Suboption Frame Format:
Suboption
Type
Length
Type
Length
MAC Address
0x02
8
0
6
6
Circuit Form:
The circuit form is a flexible architecture. It allows user to combine any information or the system
configurations into the circuit sub-option.
The Circuit Form is a string format. And its maximum length is 100 characters.
The keyword, %SPACE, will be replaced with a space character.
The other keywords get system configurations from the system and then replace the keyword and
its leading code in the Circuit form. Eventually, the content of the circuit form is part of the
payload on the DHCP option 82 packet.
