notes: IP Phone Trust and CoS Extend

- ports are configured to trust the QoS marking only if the presence of a Cisco IP Phone is
sensed via CDP messages.
- If no Cisco device is detected on the port then the QoS markings are not trusted, even if the port is configured for trust.
- the switch may also instruct the IP Phone’s switch to apply specific CoS markings for frames received from the connected PC. The switch may either accept (trust) 802.1p bits received from the attached PC or enforce the instructed value. This feature particularly
makes sense to be used with the dot1p Voice VLAN option.

config-set:

interface FastEthernet0/6
mls qos trust cos
mls qos trust device cisco-phone
switchport priority extend cos 1


verification:

show mls qos interface #

notes: Voice VLAN

Voice VLAN (VVLAN)
- Most Cisco phones have a built-in 3-port switch and is able to distinguish the phone and the PC using different VLANs and optionally 802.1p COS.
- Voice config is communicated via CDP to the IP phone.

3 different connecting options:

1. Separate DATA VLAN / VOICE VLAN.
- VOIP frames are tagged with COS 5.
- Connection between switch and IP phone is a 802.1q trunk with native VLAN equal to data VLAN.

config set:

interface FastEthernet0/2
switchport access vlan 146
switchport voice vlan 600
spanning-tree portfast
!
interface FastEthernet0/4
switchport trunk encapsulation dot1q
switchport trunk native vlan 146
switchport trunk allowed vlan 146,600
switchport mode trunk
switchport voice vlan 600
spanning-tree portfast trunk
spanning-tree bpdufilter enable

Note that as soon as the switchport voice vlan command is applied to the
port, the spanning-tree portfast feature is automatically enabled.


2. Single VLAN for both VOICE and DATA
- Frames are not tagged, thus the phone merely acts as a switch.
- Connection between switch and IP phone is configured as a ACCESS link.

if no "switchport voice vlan" configured, then option 2 automatically applies.

3. Single VLAN for DATA and VOICE but with COS 5 marking

- ATA traffic is marked as COS 0 within a 802.1q header.
- VOICE traffic is marked as COS 5 within the 802.1q header.
- COS zero will be accepted as the access VLAN.

config set:

interface FastEthernet0/6
switchport access vlan 146
switchport voice vlan dot1p