VLAN ID Discovery over DHCP

◄ How To

With the introduction of the feature VLAN ID Discovery the following changes have to be made if you want to use this feature.

General

VLAN ID Discovery is not available when using static IP address configuration, also when setting VLAN Discovery to DHCP the phone will use the manual configured VLAN ID. The DHCP process takes longer time when DHCP Discovery is set to DHCP at the phone.

VLAN ID Discovery works this way:

1. First the Application sends out a DHCP Discover message to find out if there is a DHCP server available.
2. If the DHCP server sends a DHCP Offer back with the Option 43 Vendor specific options in a special format the phone will accept the Offer, sends back the DHCP Request and saves the VLAN ID provided by the DHCP server in the Vendor specific options.
3. After the DHCP server has send the ACK message to the phone, the phone will release the leased IP address and start a new DHCP Discover cycle with using the now known VLAN ID tagging. For this process the vendor specific option is not needed.

After this process is done the phone will start normal.

Example configuration

• Untagged Subnet is 192.1.22.0 and tagged (with VLAN ID 20) it is 10.1.22.0.
• The optiPoint 420 should get the VLAN ID 20 via VLAN Discovery.

Please make sure that the untagged packets of the first DHCP Discover cycle reach the DHCP server that provides the vendor specific options for the optiPoint. Most likely you will have to use Relay Agents/DHCP Helper.

The options for the 192.1.22.0 subnet have to contain the option 43.

The data format of the option 43 is following:

• Option Code for the first encapsulated option: 01 (always)
• Length of the first data field: 0A (always 10 Byte)
• Encapsulated data for option code 1: 53 69 65 6D 65 6E 73 00 00 00 (always Siemens plus 3 Fill bytes)
• Option Code for the second encapsulated option: 02 (always)
• Length of the first data field: 04 (always 4 Bytes)
• Encapsulated data for option code 2 (VLAN ID): 00 00 00 14 (Example for VLAN ID 20)
• Ending with 5 Padding Bytes and the end option: 00 00 00 00 00 FF (always)

If this data format for option 43 is not matching or not send by the DHCP server the phone will start with the manual configured VLAN ID, if Layer 2 QoS is enabled. If Layer 2 QoS is not enabled the phone will start without using a VLAN ID.

File:dhcp-option43.gif

Values for option 43 using a German Windows 2000 DHCP server, as described in the configuration example.