H3C S7500 Series Operation Manual page 643

Operation Manual – DHCP
H3C S7500 Series Ethernet Switches
agent. Generally, sub-option 1 and sub-option 2 must be used together to identify
information about a DHCP source.
Sub-option 2: A sub-option of option 82. Sub-option 2 represents the remote
agent ID, namely Remote ID. It holds the MAC address of the DHCP relay agent,
and is usually configured on the DHCP relay agent. Generally, sub-option 1 and
sub-option 2 must be used together to identify information about a DHCP source.
III. Related specification
The specifications concerning option 82 supporting are as follows:
RFC2131 Dynamic Host Configuration Protocol
RFC3046 DHCP Relay Agent Information Option
IV. Mechanism of option 82 supporting on DHCP relay agent
The procedure for a DHCP client to obtain an IP address from a DHCP server through
a DHCP relay agent is similar to that for the client to obtain an IP address from a
DHCP server directly. The following are the mechanism of option 82 supporting on
DHCP relay agent.
1) A DHCP client broadcasts a request packet when it initiates.
2) The DHCP relay agent on the local network receives the request packet, and
then checks whether the packet contains option 82 and processes the packet
accordingly.
3) If the packet contains option 82, the DHCP relay agent processes the packet
depending on the configured policy (that is, discards the packet, replaces the
original option 82 in the packet with its own, or leaves the original option 82
unchanged in the packet), and forwards the packet (if not discarded) to the
DHCP server.
4) If the packet does not contain option 82, the DHCP relay agent adds option 82 to
the packet and forwards the packet to the DHCP server. The forwarded packet
contains the port number of the switch to which the DHCP client is connected,
the VLAN to which the DHCP client belongs, and the MAC address of the DHCP
relay agent.
5) Upon receiving the DHCP request packet forwarded by the DHCP relay agent,
the DHCP server stores the information contained in the option field and sends a
packet that contains DHCP configuration information and option 82 to the DHCP
relay agent.
6) Upon receiving the packet returned from the DHCP server, the DHCP relay
agent strips option 82 from the packet and forwards the packet with the DHCP
configuration information to the DHCP client.
Chapter 3 DHCP Relay Agent Configuration
3-3