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Appendix D.
DHCP Relay
The WHG-505 supports DHCP Relay defined according to RFC 3046. For scaling reasons, it is advantageous to
set up an external DHCP server apart from using the internal DHCP server implemented in the WHG-505 for
assigning IP. When client-originated DHCP packets are forwarded to a DHCP server, a new option called the
"Relay Agent Information option" is inserted by the DHCP relay agent. External DHCP servers that recognize the
Relay Agent Information option may use this information to implement IP address or other parameter assignment
policies. The external DHCP server will echo the option back to the relay agent in server-to-client replies, and
strip-off the option before forwarding the reply to the client..
A graphic example of connecting 2 gateways with an external DHCP server:
Please note that the Router and Gateway 1 connected to the DHCP Server have to be under the same network
segment as the DHCP Server.
When a client requests IP address from Gateway 1 Public LAN through the build-in DHCP relay agent of the
WHG-505, the DHCP server will receive a DHCP REQUEST packet with Option 82 (a code defined in RFC 3046).
A Circuit ID will be sent by the WHG-505 when the DHCP relay is enabled to define where the packet is sent from,
and this Circuit ID will have a format of MAC_IP, such as 00:E0:22:DF:AC:DF_172.30.1.254. When the external
DHCP server gets the request packet, it will therefore know where to reply to and which IP to assign.
Here is an example of configuration file of the DHCP server:
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