We were having trouble connecting our Polycom phone up to cisco switches. Even after adding in the options in DHCP to specify the phones, they would not come up on the voice VLAN.
Best Practices are to use telephones (Cisco 79xx, Polycom CX and VVX series, etc.) with dual LAN ports on them, where one LAN port is dedicated to the connection to the Power over Ethernet (PoE) Switch, and the second LAN port is provided to make a PC or Extra Device connection from the back of the phone. This not only saves ports on the Access L2 switches, it also provides for cleaner Quality of Service (QoS) implementationn on the Voice and Video networks.
Proper Switch providioning. Discovery by the phones of the dual port capability requires LLDP (Link Layer Discovery Protocol) which is ideal, or alternatively by DHCP Option 43, suboption 10 (format 0a 02 <VLAN High Byte> <VLAN Low Byte>) for the Voice Network VLAN.
Polycom SoundPoint Phones respond to LLDP appropriately, without any special programming; but, the CX and VVX series have a strange firmware “bug” which requires the following to be set on each Cisco Access switch that phones attach to (perform this for all Cisco switches):
int range gi1/0/1-15 !!! Use the Range from your switch for all intended voice ports. Perform multiple times for all !!!
lldp med-tlv-select power
lldp med-tlv-select network-policy
(This setting script turns on LLDP, informs all ports for phones to exchange the extra LLDV-MED packets which contain VLAN to keep the CX500’s happy)
Jason Howe, PEI