C
HAPTER
44
| DHCP Commands
DHCP Relay Option 82
– 1392 –
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 902) and DHCP Relay
Information Option 82 cannot both be enabled at the same time.
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