Help configuring Voice VLANS

MrXpert

I have read some stuff about configuring Voice VLANS and just need to clarify some stuff. Hope you can help please. The IP phone is connected to fa0/1 and the PC is connected to the phone.

If I want to create a voice vlan and have it seperate from the data I use the following commands

int fa0/1
switchport mode access
switchport access vlan 10
switchport access voice vlan 20
spanning-tree portfast
cdp enable

Doing show vlan shows that both vlan 10 and 20 were created and vlan 20 is stated as being a voice vlan. So clearly this appears to work...or does it?

but I read on Chris Bryant's guide that by using switchport mode access the data and voice will be combined into a single vlan and this causes jitter and that the interface fa0/1 should be configured as a trunk instead. he mentioned using switchport voice vlan dot1p option as this puts voice traffic into native vlan 0 and makes it high priority.Or I could use switchport voice vlan 20 and this makes it a dot1q trunk but then what happens to the data traffic to the PC in VLAN 10 as doesn't this need to be an access port? a switchport cannot be a trunk port and an access port at same time can it?

I have looked at Cisco's website to get some understanding of it but it's confused me. I have never done anything with voice so perhaps this is my major failure and it is just a lack of understanding clearly by me.
Can anyone explain this please.is there a best practice? Much appreciated your help and advice.

malcybood

If you use the following commands then voice and data will be on the same VLAN (missing switchport voice vlan command)

int fa0/1
switchport mode access
switchport access vlan 10
spanning-tree portfast
cdp enable

When you add the switchport voice vlan 20 command the switch then communicates with the phone via CDP and ensures packets from the IP phone are put onto the voice VLAN.

It used to be required to set the switch port to a trunk port and configure native VLANs etc when using non Cisco IP phones in Cisco switches. This was because the voice VLAN negotiation is performed during CDP discovery process and CDP is Cisco proprietary.

This can be overcome these days for non Cisco IP phones with current switches by using a protocol called LLDP.

LLDP is basically an open standards version of CDP but wasn't available feature on older switches such as 2950 etc, so it required the port to be configured as a trunk, data VLAN being set as native VLAN and the voice DHCP scope was configured to "tell" the IP phone to obtain an IP on the voice VLAN. Very laborious.

The above is overcome with CDP and LLDP.

For example I've deployed Cisco switch estates with NEC and Avaya IP phones, where the ports use switchport voice vlan command and LLDP ensures that voice and data is separated.

Cisco refer to the voice vlan as the "auxiliary VLAN" in case you see that terminology and are confused. Aux and voice VLAN are generally the same thing when speaking about voice vlans on Cisco switches.

7of9

Just to echo malybood, I had issues with Nortel phones with the switchport voice vlan command until I enabled LLDP on the port, then everything worked fine. There is no need to configure a trunk port for the phone, even if the phones are not Cisco.

Voice vlan ports are one of those gray areas. Technically, it allows a port to pass more than one vlan, which is similar to a trunk. However, it does not allow that port to pass any more than the voice and data vlan it is configured for and it allows you to configure a port so that if someone decides to plug in a phone between where a PC was plugged in and the jack, no additional configuration is necessary. It is also more secure than just opening up a trunk port that anyone could come in and plug into and trunk to. So, yes, a voice vlan port is a better idea than a trunk port for your phones, as long as both the switch and the phone support either CDP or LLDP.

MrXpert

Hey thanks to you both for the clarification!:):)