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VLAN configuration guidelines

Last updated on May 11, 2022

1. Introduction

2. General VLAN configuration guidelines

3. DHCP server voice VLAN provisioning

4. Phone voice VLAN DHCP request-response process

5. DHCP option for assigning a provisioning service

6. Phone boot process

7. Wireshark trace of the DHCP discovery process

1. Introduction

This document provides VLAN configuration guidelines for IP hardphones (deskphones and conference room phones) deployed at enterprise sites.

This article refers to domain names for the IP phone provisioning server. You can find this domain name in the network requirements document in the table for desk and conference phones.

2. General VLAN configuration guidelines

The following configuration guidelines assume that your hardphone can operate on a separate, voice-tagged VLAN. Your phone’s Ethernet switch must be properly configured to use VLANs.

When a phone boots, it attempts to discover the voice VLAN dynamically by using these discovery protocols in the following order:

LLDP: Link Layer Discovery Protocol
CDP: Cisco Discovery Protocol
DHCP: Dynamic Host Configuration Protocol
Static phone configuration: not addressed below

Once your phone discovers the VLAN, it sends out a DHCP request to the VLAN using Option 160. Hardphones typically execute the following process to receive voice VLAN information:

Upon booting, the phone requests a voice VLAN ID via an LLDP message that is broadcast in the local network.
If the phone receives a LLDP response message with the VLAN ID, it sends a DHCP request for the IP address and Option 160 on the voice-tagged VLAN ID.
If the phone doesn’t receive a LLDP response message, it sends a CDP broadcast message to request a voice VLAN ID.
If the phone receives a CDP response message with the voice-tagged VLAN ID, it sends a DHCP request for the IP address and Option 160 on the voice-tagged VLAN.
If the phone doesn’t receive a CDP response message, the phone sends a DHCP request for the IP address and voice VLAN DHCP Option on the data VLAN to the DHCP server.

3. DHCP server voice VLAN provisioning

To configure a voice VLAN, you must provision the following items on your DHCP server:

If you use DHCP to configure voice VLAN, you must ensure that LLDP/CDP is not used in the same broadcast domain.
The phone’s required DHCP options must be defined in the data VLAN scope of the DHCP server. The required DHCP options are:

1 Subnet Mask
3 Router (default gateway) IP address
6 Domain Name Server (DNS) IP address

You have four DHCP Options for configuring a data VLAN in the DHCP data scope:

If one option doesn’t work, move down the list to try the others.

You must configure your VLAN in the format VLAN-A=x;
- VLAN and A must be capitalized.
- x = voice VLAN number
- You must include the semicolon after the x.
  
  Example: VLAN-A=10; sets the voice VLAN to 10.
Define a voice VLAN scope on the DHCP server with Options 1, 3, and 6.
Optionally, the provisioning server domain name can be configured in Option 160 in the DHCP voice scope.

4. Phone voice VLAN DHCP request-response process

The VLAN request-response process is as follows:

The IP phone performs a DHCP request on the Default VLAN (No VLAN tagging).
The DHCP server responds with the option to set VLAN. (Example: VLAN-A=10; sets the voice VLAN to 10.)
The IP phone releases the previous DHCP IP address.
The IP phone reboots after receiving the VLAN option.
The IP phone requests voice VLAN DHCP scope (with VLAN tag = 10).
The DHCP server responds with a new IP address for the voice VLAN.
(Optional: Use Option 160 with your phone provisioning server domain name.)
The phone continues the boot process.
The phone attempts to contact a provisioning server on the voice VLAN.

5. DHCP option for assigning a provisioning service

Before using any IP hardphone, you must provision it via the RingCentral Admin Portal to specify the phone’s MAC address. There are two ways to provision a phone:

Your existing phone has already been configured with the provisioning server’s domain name.
- After it receives the IP and DNS server addresses via DHCP, the phone will connect to the provisioning server to complete the provisioning process.
Your phone has not yet been configured to connect to the appropriate provisioning server. In this case, you can use DHCP to obtain the domain name for the provisioning server.
- Create DHCP Option 160 on the DHCP server for the scope servicing the IP phone.
- Set DHCP Option 160 to an ASCII string equal to the domain name of the provisioning service.
- Perform a factory reset of the phone.

6. Phone boot process

The generic phone boot process may implement several configuration protocols:

LLDP to configure Voice VLAN: See switch manufacturer documentation to configure LLDP for the voice VLAN.
CDP to configure voice VLAN: See switch manufacturer documentation to configure CDP.
DHCP: Use Options 128, 144, 157, or 191 to configure the voice VLAN.
Configure VLAN-A=10 on the DHCP server and Ethernet switches to set the voice VLAN to 10. (See Section 4 for further instructions on this step.)
Enter the option value in the data DHCP scope.
DHCP Option 160: Configure the domain name of the provisioning service.
Enter the option value in the voice DHCP scope.

If you need more information about any of these processes, you may need to contact your phone’s manufacturer.

7. Wireshark trace of the DHCP discovery process

A sample Wireshark packet capture file provides the following details of a Polycom phone DHCP discovery process:

Frame 1: IP Phone DHCP Discover: Parameter request asking for Options 191, 157, 144, 128, and 160.
Frame 2: DHCP Offer: Option 160 (provisioning server) and 144 (VLAN Info) being returned.
Frame 5: IP Phone sending DHCP Release: Release of old IP address on Data VLAN.
Frame 6: IP Phone DHCP Discover on Voice VLAN: New Discover on the Voice VLAN requesting Option 160.
Frame 7: DHCP Offer: Option 160 for the provisioning server.

Other phones will produce different protocol message exchanges.