Tech Support: 1.888.678.9427 - Comnet CWGE24MS2 Installation And Operation Manual

INSTALLATION AND OPERATION MANUAL
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
82 N
The Agent Information field consists of a sequence of SubOpt/Length/Value tuples for each sub-
option, encoded in the following manner:
Sub-
Option
1
DHCP Agent
Sub-option Code
---------------
1
2
Circuit ID Sub-option Format:

TECH SUPPORT: 1.888.678.9427

Agent Information Field
i1 i2
Len Sub-Option Value
N s1
Sub-Option Description
----------------------
Agent Circuit ID Sub-option
Agent Remote ID Sub-option
Suboption Length Information
Type
0x01
i3 i4
s2 s3 s4
Circuit Form
CWGE24MS2
. . . iN
. . . sN
INS_CWGE24MS2_REV–
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