QTECH
Software Configuration Manual
11-155
be used : when networks have a single DHCP server that provides IP addresses for multiple subnets. When a router
for such a subnet receives a DHCP broadcast, it converts it to unicast (with a destination MAC/IP address of the
configured DHCP server, source MAC/IP of the router itself). The GIADDR field of this modified request is
populated with the IP address of the router interface on which it received the original DHCP request. The DHCP
server uses the GIADDR field to identify the subnet of the originating device in order to select an IP address from the
correct pool. The DHCP server then sends the DHCP OFFER back to the router via unicast. The router then converts
the DHCP OFFER back to a broadcast, sent out on the interface of the original device.
Caution :
DHCP messages are usually broadcast packets. So to use DHCP to allocate IP
for hosts in a three-level architectured network, there need be a DHCP server in every broadcast
domain. In a three-level architectured network constructed with QTECH QSW-3500 or
QSW-3900, a DHCP server is put in each VLAN. This is a greate waste of resources.A solution to
this is to use the DHCP relay feature of QTECH QSW-3900, which relays DHCP messages to
DHCP servers.Thus only one DHCP server is needed at least.
Since the packets are broadcasted in the process of obtaining IP addresses, DHCP is only applicable to the
situation that DHCP clients and DHCP servers are in the same network segment, that is, you need to deploy at least
one DHCP server for each network segment, which is far from economical.
The DHCP relay agent is designed to address this problem. It enables DHCP clients in a subnet to
communicate with the DHCP server in another subnet so that the DHCP clients can obtain IP addresses. In this case,
the DHCP clients in multiple networks can use the same DHCP server, which can decrease your cost and provide a
centralized administration.
11.5.2
DHCP Relay Agent Fundamentals
Figure below illustrates a typical DHCP relay agent application.
Typical DHCP relay agent application
DHCP relay agents can transparently transmit broadcast packets on DHCP clients or servers to the DHCP
servers or clients in other network segments.
In the process of dynamic IP address assignment through the DHCP relay agent, the DHCP client and DHCP
server interoperate with each other in a similar way as they do without the DHCP relay agent. The following sections
only describe the forwarding process of the DHCP relay agent. For the interaction process of the packets, see
Obtaining IP Addresses Dynamically
.
1) The DHCP client broadcasts the DHCP-DISCOVER packet.
2) After receiving the packets, the network device providing the DHCP relay agent function unicasts the packet
to the designated DHCP server based on the configuration.
