Dhcp Option 82 - TRENDnet TI-PG1284i User Manual

TRENDnet User's Guide

DHCP Option 82

DHCP Option 82 is the "DHCP Relay Agent Information Option". Option 82 was designed
to allow a DHCP Relay Agent to insert circuit specific information into a request that is
being forwarded to a DHCP server. Specifically the option works by setting two sub-
options: Circuit ID and Remote ID.
The DHCP option 82 is working on the DHCP snooping or/and DHCP relay.
The switch will monitor the DHCP packets and append some information as below to the
DHCPDISCOVER and DHCPREQUEST packets. The switch will remove the DHCP Option 82
from the DHCPOFFER and DHCPACK packets. The DHCP server will assign IP domain to the
client dependent on these information.
The maximum length of the information is 32 characters.
In residential, metropolitan Ethernet-access environments, DHCP can centrally manage
the IP address assignments for a large number of subscribers. When the DHCP option-82
feature is enabled on the switch, a subscriber device is identified by the switch port
through which it connects to the network (in addition to its MAC address). Multiple hosts
on the subscriber LAN can be connected to the same port on the access switch and are
uniquely identified.
When you enable the DHCP snooping information option 82 on the switch, this sequence
of events occurs:

The host (DHCP client) generates a DHCP request and broadcasts it on the
network.

When the switch receives the DHCP request, it adds the option-82 information in
the packet. The option-82 information contains the switch MAC address (the
remote-ID suboption) and the port identifier, vlan-mod-port, from which the packet
is received (the circuit-ID suboption).

If the IP address of the relay agent is configured, the switch adds the IP address in
the DHCP packet.

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,
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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
82 N i1
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
1 N s1
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 Length
Type
0x01
TI-PG1284i
Agent Information Field
i2 i3 i4 . . .
Sub-Option Value
s2 s3 s4 . . .
Information
Circuit Form
iN
sN
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