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6.4.
DHCP Option (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
.
Option Frame Format:
Code
Len
Agent Information Field