Moxa EDS-408A-1M2S-SC-T [85/109] Configuring dhcp relay agent

Moxa Managed Ethernet Switches Featured Functions

3-73

Configuring Set Device IP

Automatic “Set Device IP” by DHCP/BootP/RARP

Desired IP Address

Setting Description Factory Default

IP Address Set the desired IP of connected devices. None

Configuring DHCP Relay Agent

The DHCP Relay Agent makes it possible for DHCP broadcast messages to be sent over routers. The DHCP

Relay Agent enables DHCP clients to obtain IP addresses from a DHCP sever on a remote subnet, or those that

are not located on the local subnet.

DHCP Relay Agent (Option 82)

Option 82 is used by the relay agent to insert additional information into the client’s DHCP request. The Relay

Agent Information option is inserted by the DHCP relay agent when forwarding client-originated DHCP packets

to a DHCP server. Servers can recognize the Relay Agent Information option and use the information to

implement IP addresses to Clients.

When Option 82 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 access switch and are uniquely identified.

The Option 82 information contains 2 sub-options, Circuit ID and Remote ID, which define the relationship

between the end device IP and the DHCP Option 82 server. The Circuit ID is a 4-byte number generated by the

Ethernet switch—a combination of physical port number and VLAN ID. The format of the Circuit ID is shown

below:

FF–VV–VV–PP

This is where the first byte “FF” is fixed to “01”, the second and the third byte “VV-VV” is formed by the port

VLAN ID in hex, and the last byte “PP” is formed by the port number in hex. For example:

01–00–0F–03 is the “Circuit ID” of port number 3 with port VLAN ID 15.