H3C S3600 Series Operation Manual page 44

Operation Manual – DHCP
H3C S3600 Series Ethernet Switches-Release 1510
RFC2131 Dynamic Host Configuration Protocol
RFC3046 DHCP Relay Agent Information Option
IV. Mechanism of option 82 supporting on DHCP relay
The procedure for a DHCP client to obtain an IP address from a DHCP server through a
DHCP relay 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.
1) A DHCP client broadcasts a request packet when it initiates.
2) If a DHCP server exists in the local network, it assigns an IP address to the DHCP
client directly; otherwise the DHCP relay on the local network receives and
processes the request packet. The DHCP relay checks whether the packet
contains option 82 and processes the packet accordingly.
3) If the packet contains option 82, the DHCP relay processes the packet depending
on the configured strategy (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 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 port belongs, and the MAC address of the DHCP relay.
5) Upon receiving the DHCP request packet forwarded by the DHCP relay, 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.
6) Upon receiving the packet returned from the DHCP server, the DHCP relay strips
option 82 from the packet and forwards the packet with the DHCP configuration
information to the DHCP client.
Note:
Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVER
packets and DHCP-REQUEST packets. As DHCP servers coming from different
manufacturers process DHCP request packets in different ways (that is, some DHCP
servers process option 82 in DHCP-DISCOVER packets, whereas the rest process
option 82 in DHCP-REQUEST packets), a DHCP relay adds option 82 to both types of
packets to accommodate to DHCP servers of different manufacturers.
Chapter 3 DHCP Relay Configuration
3-3