Switchport Voice VLAN – What Does It Do?
I hope this was a little more insight into what the magical command switchport voice vlan does on a switch. I’m often asked by people new to voice why this must be configured each time. Before I blindly regurgitated lines like “special 802.1q trunk” and “do it or it won’t work.” Now I have a very interesting story to tell and threaten people with if they don’t do it.
In the 3500xl days, you had to configure the voice vlan and still configure the port as a trunk. The former only modified CDP’s advertisements. You then had to make sure that you pruned off the non-desktop and voice vlans or you’d wind up with some very noisy ports and a security problem.
Do you know if non Cisco phones can understand CDP? We are rolling out Polycom phones and they phones are ending up in the data Vlan instead of the voice Vlan. Not sure if its because they dont understand CDP or maybe its something else? I honestly thought LLDP is what told the phone which Vlan it’s in, power settings, etc.
We started out with Nortel IP phones, and have more recently started deploying Polycoms. As I understand it, when you connect a Nortel phone to a port with voice vlan defined, it actually makes a DHCP request on the access VLAN and we had to configure our access VLAN DHCP scope with option 128 (Nortel-i2004) which specifies the Voice VLAN the phone should use. The phone then hops onto the Voice VLAN and makes a second DHCP request and this is where it actually pulls it’s address. Then in the Voice VLAN scope I have option 128 and 144 configured as well. When we started rolling out the PolyCom phones they seemed to hop on the correct VLAN as well, but I don’t know for sure if they are doing the same thing the nortel phones are doing or not. I’ve never done packet captures to try to determine that.. They work though. I can say that.
We use Siemens phones and the voice Vlan configuration, but the Vlan ID is given to the phone by our DHCP servers using option 43, not LLDP/CDP.
Most Polycom phones do actually support CDP. You shouldn’t need to configure anything more than the switchport voice VLAN command.
I haven’t had any luck getting the phone to register on the voice VLAN with CDP. Are you aware of a min level of IOS code for this to work?
Don’t forget about switchport port security. If you setup port security to limit the number of mac addresses on a switchport, you will need to take into account the switchport in the phone. It has it’s own mac address and will cause issues if you set the max to 2 instead of 3.
Yes!!! I ran into this issue the other day. I’m seeing the phone register its MAC on the data and voice VLAN, so I had to set the port security max to 3 to keep the port from err-disabling. Why would the phone’s MAC be registered to both VLANs?
Also, don’t forget about QoS. mls qos trust cisco-phone pushes the trust boundary out to the phone and makes the phone do the QoS marking for phone traffic. Additionally, in CUCM you can tell the phones to either believe or remark CoS values entering their pc port. Simply put, the phones will remark pc traffic with default CoS values if you so choose. This is important if an application sends it’s traffic with better than class 0 markings.
Thanks for the information. I have noticed VoiceVLAN and thought may be some fixed vlan no. that will be using 802.1q tagging.
I would think that the traffic WOULD get to the switch but it would get there “untagged”. What is untagged to a switch? It is “Native VLan” traffic. Have you actually tested this statement in a lab?
just to add a little input. I recently deployed some 3560’s and some cisco 6921’s. We couldn’t get the phones to pull the tftp address and get it’s load. found out we had to remove the voice vlan and then it registered fine with CUCM. We then added the voice vlan back and there were no problems. the reason is, the 6921’s use lldp-med and the switches we were using did not support lldp-med and the 6921’s do not use cdp. go figure.
Hope my understood part is correct as this is the final theory i came to conclusion on after all research on Voice/Data .