by lunarg on February 20th 2017, at 14:53

When provisioning non-Cisco phones on a Cisco-switched network, you may notice that the configured voice VLAN is not correctly provisioned to the phone. The phone ends up in the data VLAN, rather than the voice VLAN even though the switch port has been correctly configured.

There are two protocols which can be used to provision the correct VLAN to your phones: Cisco Discovery Protocol (CDP) and Link-Layer Discovery Protocol (LLDP). CDP is a proprietary protocol and is only supported on Cisco-switches and Cisco-phones. Non-Cisco devices usually use LLDP, which is an open standard supported by most other vendors. Although Cisco switches also support LLDP, it is by default not enabled, resulting in the phones not being provisioned correctly. In order to enable LLDP and allow voice VLAN provisioning, you need to configure the ports correctly (similar when using Cisco-phones) and enable LLDP to allow the phones to get the correct VLAN tag for voice VLAN.

To enable LLDP:

# configure terminal
(conf)# lldp run
(conf)# end

This enables LLDP using the default settings, which is sufficient for our needs.

When using QoS, additional configuration may be needed to trust the DSCP/COS values set by the phone. Check whether the following is present on the ports on which non-Cisco phones are connected:

auto qos voip trust
mls qos trust cos

If the following is present, it needs to be removed (reconfigure the port with "no"):

mls qos trust device cisco-phone


It is common practice (and usually a good idea) to segregate VoIP traffic and regular data traffic in separate VLANs. A lot of the VoIP hardphones have a feature allowing to use the phone as a switch for the computer which can be connected to it. The phone also supports VLAN tagging in which the voice traffic is in one VLAN and the computer connected to the phone is in another. The process of assigning the voice VLAN to the phone can be automated through CDP (Cisco-proprietary) or LLDP (open standard). Most non-Cisco switches do not support CDP (as it is proprietary) and use LLDP advertising. On a Cisco switch, CDP is used by default, which is only supported on Cisco-phones. Other brands (Yealink, Snom, etc) only use LLDP, and thus, auto-configuration fails.

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