Chapter 25
| DHCP Commands
DHCP Relay Option 82
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â—† 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.
â–
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.
â–
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:
â–
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.
â–
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.
â—† 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
:
â–
There is no DHCP relay server set on the switch, when the switch receives a
DHCP packet.
â–
A DHCP relay server has been set on the switch, when the switch receives a
DHCP request packet with a non-zero relay agent address field (that is not
the address of this switch).
â–
A DHCP relay server has been set on the switch, when the switch receives
DHCP reply packet without option 82 information from the management
VLAN.
â–
The reply packet contains a valid relay agent address field (that is not the
address of this switch), or receives a reply packet with a zero relay agent
address through the management VLAN.
â–
A DHCP relay server has been set on the switch, and the switch receives a
reply packet on a non-management VLAN.
â—† Use the ip dhcp relay information policy command to specify how to handle
DHCP client request packets which already contain Option 82 information.
â—† DHCP Snooping Information Option 82 (see page 272) and DHCP Relay
Information Option 82 cannot both be enabled at the same time.
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