Dhcp Relay Option 82 - WAGO 852-1305 Manual

78 Enhanced Features
7.2.5

DHCP Relay Option 82

"DHCP Option 82" ("DHCP Relay Agent Information Option"). Option 82 was
designed to allow a "DHCP Relay Agent" to insert circuit-specific information
into a request that is being forwarded to a DHCP server. Specifically, the option
works by setting two sub-options: "Circuit ID" and "Remote ID".
"DHCP Option 82" operates on the basis of "DHCP Snooping" or/and "DHCP
Relay".
The switch monitors the DHCP packets and append some information under
"DHCPDISCOVER" and "DHCPREQUEST" packets. The switch deletes
"DHCP Option 82" from the "DHCPOFFER" and "DHCPACK" packets. The
DHCP server then assigns an IP domain to the client based on this information.
The maximum length of the information is 32 characters.
In residential, metropolitan ETHERNET-access environments, DHCP can
centrally manage the IP address assignments for a large number of subscribers. If
the "DHCP Option 82" function is enabled on the switch, a subscriber device is
identified by the switch port through which it connects to the network (in addition
to its MAC address). Multiple hosts on the subscriber LAN can be connected to
the same port on the switch and are uniquely identified.
If you enable "DHCP Snooping Information Option 82" on the switch, the
sequence of events is:
• The host (DHCP Client) generates a DCHP request and broadcasts it on the
network.
• If the switch receives the DHCP request, it adds the "Option 82"
information in the packet. The information contains the switch MAC
address (the "Remote ID" sub-option), "Port Identifier" and "VLAN-Mod-
PORT", from which the packet is received (the "Circuit ID" sub-option).
• If the IP address of the "Relay Agent" has been configured, the switch adds
the IP address in the DHCP packet.
• The switch forwards the DHCP request that includes the Option 82 field to
the DHCP server.
• The DHCP server receives the packet. If the server is Option 82 capable, it
can use the "Remote ID", "Circuit ID" or both to assign IP addresses and
implement policies, such as restricting the number of IP addresses that can
be assigned to a single "Remote ID" or "Circuit ID". The DHCP server then
echoes the Option 82 field in the DHCP reply.
• The DHCP server forwards the reply to the switch as a unicast if the request
was relayed to the server by the switch. If the client and server are on the
same subnet, the server broadcasts the reply. The switch verifies the Option
82 data originally entered by checking the "Remote ID" and "Circuit ID"
fields. The switch deletes the Option 82 field and forwards the packet to the
switch port that connects to the DHCP client that sent the DHCP request.
WAGO-ETHERNET-Zubehör 852
852-1305 8/4-Port 100BASE-T/1000BASE-SX/LX
Manual
1.1.0