Web Management Guide-R05

Table Of Contents
Chapter 15
| IP Services
Dynamic Host Configuration Protocol
– 559
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 it.
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 packets 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
:
There is no DHCP relay server set on the switch, when the switch receives a
DHCP packet.
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).
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.
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.
A DHCP relay server has been set on the switch, and the switch receives a
reply packet on a non-management VLAN.
DHCP relay configuration will be disabled if an active DHCP server is detected
on the same network segment.