Principle Of Dhcp Relay; Option 82 Support On The Dhcp Relay - 3Com 3C13636 Configuration Manual

Router 3000 ethernet family

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3Com Router 3000 Ethernet Family

Configuration Guide

9.3.1 Principle of DHCP Relay

The following figure illustrates DHCP relay networking.

Figure 9-5 Network diagram for DHCP relay

DHCP relay works on this principle:

When DHCP client starts and runs DHCP initialization, it sends configuration

request packet to the local network.

If there is a DHCP server in the network, it begins DHCP configurations without

If not, the local network processes the packet and forwards it to the specified

DHCP server in another sub-net.

The DHCP server undertakes configurations according to the information from the

client and sends the configuration information through DHCP relay back to the

client. Till now, dynamic configuration for the client ends is completed. In practice,

multiple message exchange processes may be required to finish client

configuration.

It can be seen that DHCP relay supports transparent transmission of DHCP broadcast

messages and can transmit broadcast messages from DHCP client (or server)

transparently to the DHCP server (or client) in another sub-net.

In real network, DHCP relay function is often implemented at a specific interface of a

router. So you should configure IP relay address for the interface to specify a target

DHCP server.

Option 82 is a relay agent information option in the DHCP packet.

When a request packet with option 82 from a DHCP client reaches a DHCP relay on its

way to the DHCP server, the DHCP relay may drop it, forward it with its option 82

information intact, or forward it with its original option 82 information replaced.

3Com Corporation

Chapter 9 DHCP Configuration

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3c13636-us - router 3036 3000 series