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Page 9 | AlliedWare™ OS How To Note: DHCP Snooping on Rapier-style switches
DHCP Option 82
Protocol details
In the DHCP packet, the Option 82 segment is organized as a single DHCP option containing
one or more sub-options that convey information known by the relay agent. The format of
the option is shown below:
Code Len Agent Information Field
+------+------+------+------+------+------+---+------+
| 82 | N | i1 | i2 | i3 | i4 | | iN |
+------+------+------+------+------+------+---+------+
The sub-options within the DHCP option are constructed as follows:
SubOpt Len Sub-option Value
+------+------+------+------+------+------+---+------+
| 1 | N | s1 | s2 | s3 | s4 | | sN |
+------+------+------+------+------+------+---+------+
SubOpt Len Sub-option Value
+------+------+------+------+------+------+---+------+
| 2 | N | i1 | i2 | i3 | i4 | | iN |
+------+------+------+------+------+------+---+------+
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:
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