2-3
Figure 2-2 Padding contents for sub-option 1 of Option 82
Figure 2-3 Padding contents for sub-option 2 of Option 82
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 support on DHCP relay agent.
1) Upon receiving a DHCP request, the DHCP relay agent checks whether the packet contains Option
82 and processes the packet accordingly.
z If the request packet contains Option 82, the DHCP relay agent 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.
z If the request packet does not contain Option 82, the DHCP relay agent adds Option 82 to the
packet and forwards the packet to the DHCP server.
2) 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.
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 agent adds
Option 82 to both types of packets to accommodate to DHCP servers of different manufacturers.