Switchport voice vlan command

[ciscobigcat]

Quick and easy questions folks:  for the switches, whenever these 2 commands are present, switchport access vlan and switchport voice vlan, how does the switch know that the traffic is Voice, let's say originating from an IP Phone, or data?

Also, are there any good links that will explain this process down to the smallest details?

thank you

[Reza Sharifi]

There is vlan id that is usually associated for each.

for example: if the the voice vlan is 10 and the data vlan is 20, when the switch gets a frame that is tagged with vlan 10 it knows that is the voice vlan.  If it is tagged with vlan 20, it knows that is coming from a data vlan.

switchport voice vlan 10

switchport access vlan 20

HTH

[LA-Engineer]

I believe that setting "switchport voice vlan" also tells the switch to send CDP messages out that port to the connected device (the phone) to use the specified vlan for its voice traffic.  All untagged frames or frames tagged with the access vlan are dropped into the access vlan.

Look up "auxiliary vlan"...

I hope I gave you correct information.

[ciscobigcat]

So the switch, whenever configured with the "switchport voice vlan" command, sends out CDP messages to the connected device?? I thought CDP was just a Cisco protocol to identify what type of Cisco device is connected to the port...

Are you implying that ALL IP Phones utilize CDP?

Again, where are the blue-prints for this?  There is nothing like having a complete understanding of what goes on under the hood and that is my quest here...

Let's say one port on a switch is configured with both commands like this?

interface FastEthernet0/4
 switchport access vlan 2
 switchport mode access
 switchport voice vlan 3
 spanning-tree portfast

The port has no devices connected to it. But then, an Avaya IP Phone gets connected to it, what is the very first thing that happens?  Does the switch initiates the talking or does the phone do this?

[LA-Engineer · Accepted solution]

I'm sorry. I'm not at a computer right now so I can't type out an in depth answer. Please see the doc at this link. http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_55_se/configuration/guide/scg3750/swvoip.html Basically, when you connect a phone, it will send out cdp hellos. The phone and the switch will know what cdp version is running on the other device. The switch will instruct the phone to use the configured "voice vlan" to tag it's voice traffic. All untagged traffic is placed into the configured "access vlan".