User's Manual

UTT Technologies Chapter 7 Advanced Setup
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other TCP/IP configuration parameters from a DHCP server. All of the physical interfaces
support DHCP client feature, and you can enable DHCP client on each interface at the
same time.
In order to meet different needs, DHCP client can use client ID to identify itself, send
DHCPREQUEST messages in broadcast or unicast mode, and require DHCP server to
respond in broadcast or unicast mode.
The Device also supports AutoIP feature, that is, if the DHCP client cannot obtain an IP
address via DHCP, it will automatically assign an IP address (in the range of
169.254.1.0/16 through 169.254.254.0/16) to itself. And the DHCP client can ascertain
that the address is not used by another host.
7.8.2.3 Introduction to DHCP Relay Agent
When acting as a DHCP relay agent, the Device can forwards DHCP messages between
DHCP servers and clients. DHCP relay agents are used to forward requests and replies
between clients and servers when they are not on the same physical subnet. Then the
DHCP clients that reside on multiple physical subnets can use the same DHCP server.
Using DHCP relay agent can help you save cost and achieve centralized management.
The following describes the basic operation principle of DHCP relay agent.
1. When starting, a DHCP client will start the DHCP initialization procedure during that a
DHCPDISCOVER message will be broadcasted on its local physical subnet.
2. If a DHCP server that resides on the local subnet and is configured and operating
correctly, the DHCP client will directly obtain configuration parameters such as an IP
address from it. In this case, no DHCP relay agent is required.
3. If a DHCP server that doesn¶t reside on the local subnet, there must be a DHCP relay
agent on the local subnet to receive the message and then generate a new DHCP
message to send to the specified DHCP server that resides on another subnet.
4. After receiving the DHCPREQUEST message, the DHCP sever will unicast a
DHCPOFFER message to the DHCP relay agent, which includes an IP address and
other configuration parameters. After receiving the DHCPOFFER message, the
DHCP relay agent will process and forward the message to the requesting client.
5. There are multiple such interactions during the configuration process.
The Device provides the parameters of Option and Policy to specify the forwarding policy
of DHCP messages. When a DHCP relay agent receives a client-originated DHCP