40-3
Figure 40-2
Padding contents for sub-option 1 of Option 82
Figure 40-3
Padding contents for sub-option 2 of Option 82
anism of Option 82 supported on DHCP relay agent
ced
Mech
The pro
ure for a DHCP client to obtain an IP address from a DHCP server through a DHCP relay
gent is similar to that for the client to obtain an IP address from a DHCP server directly. The following
chanism of Option 82 support on DHCP relay agent.
1) Upon receiving a DHCP request, the DHCP relay agent checks whether the packet contains
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
DHCP
client.
a
are the me
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.
from the packet and forwards the packet with the DHCP configuration information to the
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
SCOVER
packet
t
request packets in different ways (that is, some DHCP servers process Option 82 in DHCP-DI
s, whereas the rest process Option 82 in DHCP-REQUEST packets), a DHCP relay agent adds
Op ion 82 to both types of packets to accommodate to DHCP servers of different manufacturers.
DHC
message from a DHCP client, the switch will replace
P Inform Message Handling Feature Used in XRN System
Some DHCP clients, after obtaining IP addresses, send DHCP Inform messages to the DHCP server to
request other configuration parameters (such as DNS server and default gateway addresses).
If an S4500 series switch belongs to a fabric, you need to enable UDP Helper and then DHCP relay
agent on it. Then, upon receiving a DHCP Inform
