CLI Reference Guide-R07

Chapter 26 | DHCP Commands (IPv4 and IPv6)
DHCP Relay (IPv4 and IPv6)
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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.
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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:
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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.
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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.
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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.
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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.
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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:
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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.
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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.
◆ DHCP packets are flooded onto the VLAN which received them if DHCP relay
service is enabled on the switch
and any of the following situations apply :
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There is no DHCP relay server set on the switch, when the switch receives a
DHCP packet.