H3C S9500 Series Operation Manual page 199

Operation Manual – Network Protocol
H3C S9500 Series Routing Switches
Configuration". The following only introduces the working mechanism of Option 82
H C P
support on DHCP relay.
1) A DHCP client broadcasts a request packet during initialization.
2) If a DHCP server exists in the local network, the DHCP client acquires an IP
address from this server directly. If not, the broadcast packet is processed by the
DHCP relay device that is connected to the local network. The DHCP relay device
will check whether Option 82 exists in the packet.
3) If Option 82 exists in the packet, the relay processes the packet according to the
configured strategy. The relay may drop the packet, replace the original Option 82
with its own Option 82, or keep the original Option 82 unchanged. Then, the relay
forwards the packet (if not dropped) to a DHCP server.
4) If Option 82 does not exist in the request packet, the DHCP relay device adds
Option 82 into the packet and then forwards it to a DHCP server. In this way, the
packet contains the MAC address and VLAN ID of the port of the switch that the
DHCP client is connected to, and the MAC address of the DHCP relay itself.
5) After receiving the DHCP request packet forwarded by the DHCP relay, the DHCP
server records the information carried by the option in the packet. Then, the DHCP
server sends to the DHCP relay a response packet which carries the DHCP
configuration information and Option 82 information.
6) After receiving the response packet sent by the DHCP server, the DHCP relay
strips Option 82 information in the packet. Then, it forwards the packet that carries
the DHCP configuration information to the DHCP client.
Note:
The request packets sent by a DHCP client fall into two types: DHCP_DISCOVER
packets and DHCP_REQUEST packets. The DHCP relay device adds Option 82 into
both types of request packets. This is because that the DHCP servers from different
manufacturers process the request packets in different ways. Some of them process
Option 82 in the DHCP_DISCOVER packets, while others process Option 82 in the
DHCP_REQUEST packets.
5.2 Configuring Option 82 Supply on DHCP Relay
5.2.1 Prerequisites
Before enabling Option 82 support on DHCP relay, you should configure:
The network parameters and the relay function on the DHCP relay
The network parameters, the parameters related to the allocation strategy such as
the address pools and the address allocation lease on the DHCP server.
Chapter 5 DHCP Option 82 Configuration
5-1