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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 hard phones (desk phones and conference room phones) deployed at enterprise sites.

References are made to the IP phone provisioning server domain name. This domain name can be found in the network requirements document in the table for desk and conference phones.

2. General VLAN configuration guidelines

The next considerations assume that the hard phone can operate on a separate voice-tagged VLAN. The Ethernet switch connected to the phone must be configured properly to use VLANs. When a phone boots, it attempts to dynamically discover the voice VLAN by using the following (discovery) protocols in the following order:

LLDP – Link Layer Discovery Protocol
CDP – Cisco Discovery Protocol
DHCP – Dynamic Host Configuration Protocol
Statically configuration on the phone (not covered below)

Once the VLAN is determined, the phone sends out a DHCP request on the VLAN using Option 160. The following process is typically executed by hard phones to receive Voice VLAN information:

When the phone boots, it requests a voice VLAN ID via an LLDP broadcast into the local network.
If LLDP responds with the VLAN ID, then a DHCP request for IP address and 160 Option is sent on the tagged voice VLAN ID.
If no LLDP response occurs, then a CDP message is broadcast requesting voice VLAN ID.
If CDP responds with voice VLAN ID, a DHCP request for IP address and 160 Option is sent on the tagged voice VLAN.
If no CDP response is received, a DHCP request is sent on the data VLAN for the voice VLAN DHCP Option and IP information.

3. DHCP server voice VLAN provisioning

To configure a voice VLAN the following items need to be provisioned on the DHCP server:

When using DHCP to configure the voice VLAN it needs to be ensured that LLDP/CDP is not used in the same broadcast domain.
DHCP Options required by the phone must be defined in the data VLAN scope of the DHCP server (click to view a Wireshark sample packet capture file) are:

1 Subnet Mask
3 Router (Default Gateway) IP Address
6 Domain Name Server (DNS) IP address

Optionally, four different DHCP options can be used to configure a data VLAN in the DHCP data scope: Options 128, 144, 157, or 191. Sometimes an option does not work and a different one has to be selected.
The VLAN must be configured as VLAN-A=x; -- where VLAN and A must be in CAPS, x = voice VLAN number and the semicolon behind x must be present.
Example: VLAN-A=10; -- sets voice VLAN to 10
Define a voice VLAN scope with the proper IP and 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 including reboots is as follows:

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

5. DHCP option for assigning a provisioning service

To use any IP hard phone, it must be provisioned in the administration service portal identifying the phone’s MAC Address. Phones can be obtained in two ways:

The purchased phone is already configured with the provisioning server domain name.
- After receiving the IP and DNS server addresses via DHCP, the phone will connect to the provisioning server to provision the phone appropriately.
The phone does not have the appropriate provisioning server configured. In that case, DHCP can be used to provide the domain name for the provisioning service:
- 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 leverage several protocols:

LLDP to configure Voice VLAN – See switch manufacturer documentation on how to configure LLDP for the voice VLAN.
CDP to configure voice VLAN – See switch manufacturer documentation on how to configure CDP.
DHCP – Option 128, 144, 157, 191 to configure voice VLAN.
VLAN-A=10 -- to set voice VLAN to 10, must be in CAPS and must end with a semicolon, no white spaces.
Place option in the data DHCP scope.
DHCP Option 160: configure the domain name of the provisioning service.
Place option in the voice DHCP scope.
More information can be found in the phone administration guide.

7. Wireshark trace of the DHCP discovery process

The sample packet capture file: polycom dhcp boot process with vlan-2.pcapng provide details of a Polycom phone DHCP discovery process. Other phones will produce protocol message exchanges.

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.