User manual

DHCP Relay Agent
6874_en_09 PHOENIX CONTACT 10-1
10 DHCP Relay Agent
The MMS is able to act as a DHCP relay agent. The DHCP relay agent function is required
for DHCP option 82. DHCP option 82 is used by the DHCP server when assigning
addresses to identify the requesting termination device via the corresponding physical
switch port. In the event of device replacement, DHCP option 82 enables the replacement
device to be assigned the same IP address as the replaced device due to the physical
position in the network. The DHCP packet (broadcast) originally sent by the termination
device also reaches the DHCP server if it is located in the same subnetwork. The DHCP
server should be set so that it ignores this packet.
Sequence:
Every time the MMS receives a DHCP discover/request that has been sent by a
termination device, the MMS extends the "DHCP option 82" field and forwards the data
packet to the specific DHCP server. The desired DHCP server should be configured in
WBM on the "Relay Agent" page.
The DHCP server can generate a response using option 82 information and can send this
to the relay agent.
The switch then removes the DHCP option 82 data from the DHCP server response and
forwards it to the termination device that triggered the request.
Information in the DHCP option 82 field:
The MMS extends the VLAN ID in the DHCP option 82 field for the VLAN to which the
termination device is assigned and the switch port to which the termination device is
connected. In addition, the MMS enters its own DHCP option 82 remote ID in the field. The
DHCP option 82 remote ID can be configured by the user and contains the IP or MAC
address of the MMS.
10.1 Activating the DHCP Relay Agent
Enable the web page via "General Configuration/User Interfaces". Activate the agent with
"Enable", specify at least the IP address of the server.
The MMS management does not start the relay agent while the switch is operating as a
DHCP client.