C
HAPTER
16
| IP Services
Dynamic Host Configuration Protocol
– 598 –
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 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 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
:
■
There is no DHCP relay server set on the switch, when the switch
receives a DHCP packet.
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