INS_CWGE24MS2_REV–
10/05/16 PAGE 125
INSTALLATION AND OPERATION MANUAL
CWGE24MS2
TECH SUPPORT: 1.888.678.9427
When you enable the DHCP snooping information option 82 on the switch, this sequence of
events occurs:
» The host (DHCP client) generates a DHCP request and broadcasts it on the network.
» When the switch receives the DHCP request, it adds the option-82 information in the packet.
The option-82 information contains the switch MAC address (the remote-ID suboption) and the
port identifier, vlan-mod-port, from which the packet is received (the circuit-ID suboption).
» If the IP address of the relay agent is configured, the switch adds the IP address in the DHCP
packet.
» The switch forwards the DHCP request that includes the option-82 field to the DHCP server.
» The DHCP server receives the packet. If the server is option-82 capable, it can use the remote
ID, the circuit ID, or both to assign IP addresses and implement policies, such as restricting
the number of IP addresses that can be assigned to a single remote ID or circuit ID. Then the
DHCP server echoes the option-82 field in the DHCP reply.
» The DHCP server unicasts the reply to the switch if the request was relayed to the server by
the switch. When the client and server are on the same subnet, the server broadcasts the
reply. The switch verifies that it originally inserted the option-82 data by inspecting the remote
ID and possibly the circuit ID fields. The switch removes the option-82 field and forwards the
packet to the switch port that connects to the DHCP client that sent the DHCP request.
Option Frame Format:
Code
Len
Agent Information Field
82
N
i1
i2
i3
i4
. . .
iN
The Agent Information field consists of a sequence of SubOpt/Length/Value tuples for each sub-
option, encoded in the following manner:
Sub-
Option
Len
Sub-Option Value
1
N
s1
s2
s3
s4
. . .
sN
DHCP Agent
Sub-Option Description
Sub-option Code
---------------
----------------------
1
Agent Circuit ID Sub-option
2
Agent Remote ID Sub-option
Circuit ID Sub-option Format:
Suboption
Type
Length Information
0x01
Circuit Form
