Introduction To Dhcp-Snooping Option - 3Com 4500 PWR 26-Port Configuration Manual

Figure 41-1 Typical network diagram for DHCP snoop
DHCP Client DHCP Client
DHCP Client DHCP Client
DHCP snooping listens the following two types of packets to retrieve the IP addresses the DHCP clients
obtain from DHCP servers and the MAC addresses of the DHCP clients:
DHCP-REQUEST packet
DHCP-ACK packet

Introduction to DHCP-Snooping Option 82

Introduction to Option 82
For details about Option 82, refer to
Padding content
There is no specifi
By default, the sub-options of Option 8
s nooping) a re padd
sub-option 1
number by 1) and VLAN ID of the port
sub-option 2
device that re
B y default, when S4500 Series Ethernet Switches serve as DHCP snooping devices, Option 82 adopts
the extended format. Refer to
(with the default padding contents). That is, the circuit ID or remote ID sub-option defines the type and
length of a circuit ID or remote ID.
The remote ID type field and circuit ID type field are determined by the option storage format. They are
both set to 0 in the case of HEX fo
Figure 41-2 Extended format of the circ
Eth1/0/1
Switch A
(DHCP Snooping)
Option 82 Sup
and frame format of
cation for w hat should
ed as follows:
(circuit ID sub-o ption) : Padded with the port index (smaller than the physic
(remote ID sub-o ption):
ceived the client's requ
Figure 41-2
rmat and to 1 in
uit ID sub-option
ing application
DHCP Server
Internet
Eth1/0/2
Switch B
(DHCP Relay)
port on DHCP Relay
Option 82
be padded in Option 82. Manufacturers can pad it as required
2 for S4500 Series Ethernet Switches (enabled with DH
that received the client's re
Padded with the bridge MAC address of the DHCP snooping
est.
and Figure 41-3 for the extended format of the sub-options
the case of ASCII format.
41-2
Agent.
quest.
.
CP
al port