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DHCP relay agent
6874_en_16
PHOENIX CONTACT
11-1
11 DHCP relay agent
The MFL SWITCH MM HSMS/MCS 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/MCS receives a DHCP discover/request that has been sent by a
termination device, the MMS/MCS 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/MCS 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/MCS 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.
11.1
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/MCS management does not start the relay agent while the switch is operating
as a DHCP client.
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