Management and Configuration Guide K/KA/KB.15.15

Configuring 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.
NOTE: 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:
(HP_Switch_name#) vlan 10
HP Switch(vlan-10)# tagged 1-10
HP Switch(vlan-10)# voice
HP 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.
Syntax
[no]cdp mode pre-standard-voice [admin-status <port-list>
[tx_rx | rxonly]]
Enable CDP-compatible voice VLAN discovery with pre-standard VoIP phones. In this mode, when
a CDP VoIP VLAN query is received on a port from pre-standard phones, the switch replies back
with a CDP packet that contains the VID of the voice VLAN associated with that port.
pre-standard-voice Enables CDP-compatible voice VLAN discovery with pre-standard
VoIP phones.
admin-status Sets the port in either transmit and receive mode, or receive mode
only.
Default: tx-rx.
<port-list> Sets this port in transmit and receive mode, or
receive mode only.
rxonly Enable receive-only mode of CDP processing.
238 Configuring for Network Management Applications