Voice VLAN provision

Back in 2007, I have used Cisco Switch to provision Mitel Voip phone, and phone will use a different VLAN than PC and share the same data drop. However, I forge the technical detail how the hell the Phone is smart enough to get a different VLAN.

Now, I know it is because CDP. Actually, the there are other options, such as DHCP VLAN provision. Polycom SIP phone support CDP as well. Here is the link.

Automatic Configuration of VLAN ID
In the Network Settings menu, there is another field called “CDP”; this controls whether the phone
supports the Cisco Discovery Protocol. Among many other things, CDP allows the Ethernet switch to
inform an IP phone what VLAN ID it should use without requiring manual configuration of each
phone.
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If CDP is enabled and a VLAN ID is also manually configured, any value received via CDP will
override the manual configuration. If no VLAN ID is received via CDP, the phone will fall back to the
configured value, if present, or to the native VLAN.
QoS Configuration
By default, the phone sends all audio packets with 802.1p tag and IP Precedence values of 5, which
is the highest priority level available for user traffic. This can be adjusted via the
qos.ethernet.rtp.user_priority and qos.ip.rtp.precedence values in the configuration files. Marking
audio packets with DSCP values is currently not supported.

More infomation avaiable at voip-info.org and this link

http://wiki.voiceroute.org/wiki/Overview_of_Auto-provisioning_Phones