CLI Reference Guide-R04

Chapter 30 | DHCP Commands
DHCP Relay Option 82
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◆ By default, the relay agent also fills in the Option 82 circuit-id field with
information indicating the local interface over which the switch received the
DHCP client request, including the VLAN ID, stack unit, and port. This allows
DHCP client-server exchange messages to be forwarded between the server
and client without having to flood them onto the entire VLAN.
◆ DHCP request packets received by the switch are handled as follows:
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If a DHCP relay server has been set on the switch, when the switch receives
a DHCP request packet without option 82 information from the
management VLAN or a non-management VLAN, it will add option 82 relay
information and the relay agent’s address to the DHCP request packet, and
then unicast it to the DHCP server.
■
If a DHCP relay server has been set on the switch, when the switch receives
a DHCP request packet with option 82 information from the management
VLAN or a non-management VLAN, it will process it according to the
configured relay information option policy:
■
If the policy is “replace,” the DHCP request packet’s option 82 content
(the RID and CID sub-option) is replaced with information provided by
the switch. The relay agent address is inserted into the DHCP request
packet, and the switch then unicasts this packet to the DHCP server.
■
If the policy is “keep,” the DHCP request packet's option 82 content will
be retained. The relay agent address is inserted into the DHCP request
packet, and the switch then unicasts this packet to the DHCP server.
■
If the policy is “drop,” the original DHCP request packet is flooded onto
the VLAN which received the packet but is not relayed.
◆ DHCP reply packets received by the relay agent are handled as follows:
When the relay agent receives a DHCP reply packet with Option 82 information
over the management VLAN, it first ensures that the packet is destined for itself.
■
If the RID in the DHCP reply packet is not identical with that configured on
the switch, the option 82 information is retained, and the packet is flooded
onto the VLAN through which it was received.
■
If the RID in the DHCP reply packet matches that configured on the switch,
it then removes the Option 82 information from the packet, and sends it on
as follows:
■
If the DHCP packet’s broadcast flag is on, the switch uses the circuit-id
information contained in the option 82 information fields to identify
the VLAN connected to the requesting client and then broadcasts the
DHCP reply packet to this VLAN.
■
If the DHCP packet’s broadcast flag is off, the switch uses the circuit-id
information in option 82 fields to identify the interface connected to
the requesting client and unicasts the reply packet to the client.