C
HAPTER
44
| DHCP Commands
DHCP Relay Option 82
– 1391 –
Otherwise, the switch’s DHCP relay agent will not be able to forward
client requests to a DHCP server.
◆ DHCP provides a relay agent information option for sending information
about its DHCP clients or the relay agent itself to the DHCP server. Also
known as DHCP Option 82, it allows compatible DHCP servers to use
this information when assigning IP addresses, or to set other services
or policies for clients.
◆ When Option 82 is enabled, the requesting client (or an intermediate
relay agent that has used the information fields to describe itself) can
be identified in the DHCP request packets forwarded by the switch and
in reply packets sent back from the DHCP server. Depending on the
selected frame format set for the remote-id by this command, this
information may specify the MAC address, IP address, or an arbitrary
string for the requesting device (that is, the relay agent in this
context).
◆ By default, the relay agent also fills in the Option 82 circuit-id field with
information indicating the local interface over which the switch received
the DHCP client request, including the VLAN ID, stack unit, and port.
This allows DHCP client-server exchange messages to be forwarded
between the server and client without having to flood them onto the
entire VLAN.
◆ DHCP request packets received by the switch are handled as follows:
■
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.
■
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.
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