Allied Telesis x900-48 series - Manual (Page 10)

Page 10 | AlliedWare™ OS How To Note: DHCP snooping on AT-9900-style switches
DHCP Option 82 > Configuring Option 82
The following table shows a list of the sub-options that are used for identifying the subscriber
that the IP address was allocated to:
Example Packet
The following shows an extract of a DHCP Request packet that includes Option 82 details:
Analysis
The following table provides an analysis of the strings in the above DHCP Request packet
extract:
The Agent circuit ID string 00 30 00 05 translates as:
30 = vlan48
05 = switch port 5
Configuring Option 82
Different commands are used to turn on Option 82 depending on whether the switch is
performing DHCP snooping or DHCP relay. For the DHCP snooping, the command is:
enable dhcpsnooping option82
The subscriber ID to be used on any given port can be set using the command:
set dhcpsnooping port=x subscriberid=”xxxx”
Sub-option RFC Description
1 RFC 3046 Agent Circuit ID sub-option – used for defining the switch port and
VLAN number of the port user(s).
2 RFC 3046 Agent Remote ID sub-option – used for defining the MAC address of
the switch that added the Option 82 information.
6 RFC 3993 Subscriber-ID sub-option – optionally configured per port using set
dhcpsnooping port=x subscriberid=x – can define port customer
name, or switch name.
DHCP Message Type = DHCP Request
Bootstrap Protocol
Option 82 – Agent Information (Option)
0000: 52 20 01 06 00 04 00 30 00 05 02 08 00 06 00 00 R ..............
0010: CD 11 B2 52 06 0C 55 73 65 72 49 64 30 31 32 33 ...R.. UserId0123
0020: 34 35 45
Text Colour Analysis
Green This is the Agent Circuit ID
Blue This is the Agent
Red This is the subscriber ID sub-option