40-2
Figure 40-1
Typical DHCP relay agent application
address assignment through the DHCP relay agent, the DHCP client and
In the process of dynamic IP
DHCP server interoperate with each other in a similar way as they do without the DHCP relay agent.
The following sections only describe the forwarding process of the DHCP relay agent. For the
interaction process of the packets, see section
Obtaining IP Addresses Dynamically
.
1) After receiving the DHCP-DISCOVER or DHCP-REQUEST broadcast from the client, the network
device providing the DHCP relay agent function unicasts the message to the designated DHCP
server based on the configuration.
2) The DHCP server selects an IP address and other parameters and sends the configuration
information to the DHCP relay agent that relays the information to the client (the sending mode is
s DHCP-DISCOVER packet, refer to section
DHCP Packet
decided by the flag filed in the client’
Format
for details).
n 82 Support on DHCP Relay Agent
troduction to Option 82
Optio
In
info
e administrator can locate the DHCP client to further
info
idual assignment policies of IP address and other parameters for the clients.
is defined, at least one sub-option must be
cuit ID sub-option)
ption 82 has no unified definition in RFC 3046. Its padding information varies with vendors. Currently,
ay agents support the extended padding
ns of Option 82 are padded as follows, as
Option 82 is the relay agent information option in the DHCP message. It records the location
rmation of the DHCP client. With this option, th
implement security control and accounting. The Option 82 supporting server can also use such
rmation to define indiv
Option 82 involves at most 255 sub-options. If Option 82
defined. Currently the DHCP relay agent supports two sub-options: sub-option 1 (cir
and sub-option 2 (remote ID sub-option).
Padding content of Option 82
O
S4500 Series Ethernet Switches that operate as DHCP rel
format of Option 82 sub-options. By default, the sub-optio
shown in
Figure 40-2
and
Figure 40-3
. (The content in brackets is the fixed value of each fiel
z
sub-option 1: Padded with the port index (smaller than the physical port numbe
of the port that received the client’s request.
z
sub-option 2: Padded with the bridge MAC address of the DHCP relay agent devi
d.)
r by 1) and VLAN ID
e that received
the client’s request.
c