Option 82 Settings

Option 82 (DHCP Relay Agent Information Option) passes port and agent information to a central DHCP server, indicating where an assigned IP address physically connects to the network. The main goal of option 82 is to help to the DHCP server select the best IP subnet (network pool) from which to obtain an IP address.

Option 82, when enabled, applies to DHCP Relay interface with IP address and DHCP Snooping. Even if Option 82 isn’t enabled, and if DCHP relay is enabled on VLAN without an IP address, option 82 information will be inserted to DHCP packets received on this VLAN.

To configure the status on the device and the format of the Option 82 data within the DHCP message, follow these steps:

Procedure

Step 1

Click IPv4 Configuration > DHCP Snooping/Relay > Option 82 Settings.

Enter the following fields:

Option 82 Insertion—Check Enable to insert Option 82 information into packets.
Numeric Token Format—Select Hexadecimal or Ascii as needed. This parameter defines the format to use for the following tokens:
- $int-ifindex$
- $int-portid$
- $switch-moduleid$
- $vlan-id$
  
  For example, the $vlan-id$ token, where VLAN ID is 35. VLAN ID 35 can be sent either as Hexa byte of 0x23 or ASCII representation of value of 0x3335. See the full information on the various tokens in the following table.

Step 2

Enter the Circuit-ID Template. Select Use Default to use the default Circuit-ID. Select User Defined to configure the Circuit-ID. Use the text box to enter the Circuit-ID template. The template is a string of free text and pre-defined tokens (see table below). You can enter tokens manually, or use the drop-down to select a token from the list of available tokens and add it to the Circuit-ID text by clicking the arrow button. Use the Preview button to view actual Sub option byte content and text representation of the selected sub-option.

Step 3

Enter the Remote-ID Template in the same way as the Circuit-ID Template, using the related text box and drop-down list.

Note

The Total Sub-Option Payload shows the dynamically updated number of reserved byte count of the payload of both sub-options. The payload must not exceed 247. Byte count is based on the reserved length of the tokens included in the sub-option, plus the number of free text chars used in the sub-option.

Step 4

Click Apply. The settings are written to the Running Configuration file.

These are the tokens that are available from the drop-down box.


Option	Description	Reserved bytes	Bytes used in Hex format	Bytes used in ASCII format
$int-ifindex$	The ifIndex of the interface on which the DHCP client request was received. Value is taken from the ifIndex field of the ifTable MIB entry	4	2	4
$int-portid$	The interface number relative to the specific unit (standalone or stacking unit). For physical interfaces this value begins with 1 for the 1st port on a specific unit, 2 for the 2nd port on that unit, until N for last port on that unit. For LAG interfaces the value is determined globally (and not based on specific unit), according to the LAG ID. For example, 1,2,3….	2	1	2
$int-name$	The full name of the interface, upon which the DHCP client request was received. The name is based on the interface full name format as used by CLI when configuring or displaying information for this interface	32	NA	Actual bytes used for the ASCII representation of the interface name (up to the limit of reserved bytes)
$int-abrvname$	The abbreviated name of the interface, upon which the DHCP client request was received. This parameter is based on the abbreviated interface name format as used by CLI when configuring or displaying information for this interface.	8	NA
$int-desc-16$	Up to 16 (first) bytes of the interface description- for the interface, upon which the DHCP client packet was received. The value for this variable is taken from the description added by the user to the interface using the interface level "description" command. Max number of bytes to use is 16 (first bytes)- even if description is longer than 16 bytes. For interfaces without a user-defined description- the interface abbreviated interface name format is used.	16	NA	Actual bytes used for the ASCII representation of the interface description (up to the limit of reserved bytes)
$int-desc-32$	Up to 32 (first) bytes of the interface description - for the interface, upon which the DHCP client packet was received. The value for this variable is taken from the description added by the user to the interface using the interface level "description" command. Max number of bytes to use is 32 (1st bytes) - even if description is longer than 32 bytes. For interfaces without user-defined description - the interface abbreviated interface name format is used.	32	NA	Actual bytes used for the ASCII representation of the interface description (up to the limit of reserved bytes)
$int-desc-64$	The full interface description (up to 64 bytes) - for the interface, upon which the DHCP client packet was received. The value for this variable is taken from the description added by the user to the interface using the interface level "description" command. For interfaces without user-defined description - the interface abbreviated interface name format is used.	64	NA
$int-mac$	The MAC address of the physical interface upon which the DHCP client request was received. The format of this field is always HEX format, with no delimiter (for example, 000000112205).	6	6	NA
$switch-mac$	The base MAC address of the device inserting the option 82 (the relay agent). The format of this field is always HEX format, with no delimiter (for example, 000000112200).	6	6	NA
$switch-hostname-16$	Up to the first 16 bytes of the device hostname.	16	NA	Actual bytes used for the ASCII representation of the hostname (up to the limit of reserved bytes)
$switch-hostname-32$	Up to the first 32 bytes of the device hostname.	32	NA
$switch-hostname-58$	The full hostname of the device.	58	NA
$switch-module-id$	The unit ID of the unit upon which the DHCP client request was received. In standalone systems ID is always equal 1.	2	1	2
$vlan-id$	The VLAN ID of the VLAN upon the DHCP client request was received. Values 1-4094	4	2	4
$vlan-name-16$	Up to the first 16 bytes of the VLAN name, for the VLAN upon which the DHCP client request was received. If a name isn’t configure for the specified VLAN, the value is taken from the relevant VLAN ifDescr MIB field of ifTable MIB entry.	16	NA	Actual bytes used for the ASCII representation of the VLAN name (up to the limit of reserved bytes)
$vlan-name-32$	The full VLAN name of the VLAN upon the DHCP client request was received. If a name is configure for the specified VLAN, the value is taken from the relevant ifDescr MIB field of ifTable MIB entry.	32	NA

Note

The total reserved byte count of the payload of both sub-options must not exceed 247. The byte count isn’t updated dynamically and shown at the bottom of the screen. Byte count is based on the reserved length (see above) of the tokens included in the sub-option, plus the number of free text chars used in the sub-option.