Voice VLAN...confused!

I've never really gave it much thought, but I'm having a hard time understanding the Voice VLAN/Access VLAN and the link between a switch/phone.

In the standard configuration of SW -> Phone -> PC, I would go in and do

switchport mode access
switchport access vlan 10
switchport voice vlan 20

Now, if I do a show interface fa x/x switchport, I see that the interface administrative/operational mode is set to access, and that the Access VLAN is 10, Voice VLAN is 20, and native VLAN is 1.

Here's where the confusion begins. I understand that the phone tags it's traffic for vlan 20, and the PC sends untagged packet, but if it's an access port, how is it carrying traffic for 2 VLANs?

If it's a "mini-trunk" as so many people likes to put it, when the PC sends its untagged packet, wouldn't those packet be placed into the port's native VLAN, which is 1?

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Comments

instant000

pham0329 wrote: »

I've never really gave it much thought, but I'm having a hard time understanding the Voice VLAN/Access VLAN and the link between a switch/phone.

In the standard configuration of SW -> Phone -> PC, I would go in and do

switchport mode access
switchport access vlan 10
switchport voice vlan 20

Now, if I do a show interface fa x/x switchport, I see that the interface administrative/operational mode is set to access, and that the Access VLAN is 10, Voice VLAN is 20, and native VLAN is 1.

Here's where the confusion begins. I understand that the phone tags it's traffic for vlan 20, and the PC sends untagged packet, but if it's an access port, how is it carrying traffic for 2 VLANs?

If it's a "mini-trunk" as so many people likes to put it, when the PC sends its untagged packet, wouldn't those packet be placed into the port's native VLAN, which is 1?

1. The IP phone has an internal switch, so this is where you can split up the traffic, from whether it goes to the phone, or the PC

2. Adding the voice vlan allows the access port to allow you to send both "voice" and "data" VLANs on the same port. By configuring it for voice VLAN, the port is configured to basically connect to the switch of an IP Phone, where the voice/data traffic would then be separated, according to which device it should go to.

This guide explains how it works:
http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_9_ea1/configuration/guide/swvoip.pdf

Chris_

Basically, ignore the vlan1 part of the output - in this case all that is saying is that IF this port were to be configured as a trunk, the native vlan would be 1. It's not a true trunk so does not use this setting, it knows to use the access vlan for untagged packets.

pham0329

instant000 wrote: »

1. The IP phone has an internal switch, so this is where you can split up the traffic, from whether it goes to the phone, or the PC

2. Adding the voice vlan allows the access port to allow you to send both "voice" and "data" VLANs on the same port. By configuring it for voice VLAN, the port is configured to basically connect to the switch of an IP Phone, where the voice/data traffic would then be separated, according to which device it should go to.

This guide explains how it works:
http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_9_ea1/configuration/guide/swvoip.pdf

I understand that there's an internal switch within the phone. What I'm confused about is the if a port is set as an access port, technically, it only belong to 1 VLAN, right?

An access port is suppose to strip off the VLAN header before forwarding the packet out the port, so how is it possible that the switch sends tagged packets to the phone?

If I were to connect a PC to a switch, and configure that port for both the Voice and Access VLAN, without having a phone in between, and assign the PC 2 IP address, one in each VLAN, would the switch still sends tagged packets (for the Voice VLAN) out it's interface?

or

If I have 2 switch connected to each other, but I configure both port as access port, and configure each port to have a voice and access vlan, will this have the same effect as configuring a trunk link with only 2 allowed VLAN?

Chris_ wrote: »

Basically, ignore the vlan1 part of the output - in this case all that is saying is that IF this port were to be configured as a trunk, the native vlan would be 1. It's not a true trunk so does not use this setting, it knows to use the access vlan for untagged packets.

That's what I figured, but I keep hearing the link between the phone and the switch referred to as a mini-trunk!...and if it's a trunk, untagged packet should be placed on the native vlan!