Dhcp Option 82 - Tripp Lite NGI-M08POE8-L2 Owner's Manual

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6.4.2 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 sub-option)
and the port identifier, vlan-mod-port, from which the packet is received (the circuit-ID sub-
option).
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, 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 unicast's 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.
Click Apply to take effect the settings.
Click Refresh to begin configuring this screen afresh.
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