CDPv2 for voice transmission

Legacy Cisco VOIP phones only support manual configuration or using CDPv2 for voice VLAN auto-configuration. LLDP-MED is not supported. CDPv2 exchanges information such as software version, device capabilities, and voice VLAN information between directly connected devices such as a VOIP phone and a switch.

When the Cisco VOIP phone boots up (or sometimes periodically), it queries the switch and advertises information about itself using CDPv2. The switch receives the VOIP VLAN Query TLV (type 0x0f) from the phone and then immediately sends the voice VLAN ID in a reply packet to the phone using the VLAN Reply TLV (type 0x0e.) The phone then begins tagging all packets with the advertised voice VLAN ID.

Configure a voice VLAN

A voice VLAN must be configured before the voice VLAN can be advertised. For example, to configure VLAN 10 as a voice VLAN tagged for ports 1 through 10, enter these commands: 

switch# vlan 10 
switch(vlan-10)# tagged 1-10
switch(vlan-10)# voice
switch(vlan-10)# exit

The switch CDP packet includes these TLVs:

  • CDP Version: 2

  • CDP TTL: 180 seconds

  • Checksum

  • Capabilities (type 0x04): 0x0008 (is a switch)

  • Native VLAN: The PVID of the port

  • VOIP VLAN Reply (type 0xe): voice VLAN ID (same as advertised by LLDPMED)

  • Trust Bitmap (type 0x12): 0x00

  • Untrusted port COS (type 0x13): 0x00

CDP should be enabled and running on the interfaces to which the phones are connected. Use the cdp enable and cdp run commands.

The pre-standard-voice option for the cdp mode command allows the configuration of CDP mode so that it responds to received CDP queries from a VoIP phone.