21 DHCP Relay Configuration
21.1 Overview
21.1.1 Understanding DHCP
The DHCP is widely used to dynamically allocate the reusable network resources, for
example, IP address.
The DHCP Client sends the DHCP DISCOVER broadcast packets to the DHCP Server. After
the DHCP Server receives DHCP DISCOVER packets, it allocates resources to the Client,
for example, IP address according to the appropriate policy, and sends the DHCP OFFER
packets. After the DHCP Client receives the DHCP OFFER packets, it checks if the
resources are available. If resources are available, it sends the DHCP REQUEST packets. If
not, it sends the DHCP DISCOVER packets. When the server receives the DHCP
REQUEST packets, it checks if the IP addresses (or other limited resources) can be
allocated. If yes, it sends the DHCP ACK packets. If not, it sends the DHCP NAK packets.
When the DHCP Client receives the DHCP ACK packets, it starts to use the resources
allocated by the server. If it receives the DHCP NAK, it may re-send the DHCP DISCOVER
packets to request for another IP address.
21.1.2 Understanding DHCP Relay Agent
The DHCP request packets have the destination IP address of 255.255.255.255. This type
of packets is only forwarded inside the subnet and is not to be forwarded by the devices. For
dynamic IP address allocation across network segments, the DHCP Relay Agent is created.
It encapsulates the received DHCP request packets into IP unicast packets and forwards
them to the DHCP Server. At the same time, it forwards the received DHCP response
packets to the DHCP Client. This way, the DHCP Relay Agent works as a transit station,
which is responsible for communicating with the DHCP Client and DHCP Server on different
network segments. Therefore, one DHCP Server in a LAN can implement the dynamic IP
management for all network segments, that is, a dynamic DHCP IP management in the
Client - Relay Agent - Server mode.
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