Voice VLAN Newbie Question

[michiganbroadband]

I'm trying to properly get my head around the concept of a Voice VLAN and how to use one.   
How is a "Voice VLAN" any different that any other VLAN?   
I mostly work with smaller switches like the SG200 SG250 series and some POE switches in the same/similar class for voice.  
But I have never employed a Voice VLAN.

My limited understanding in these types of switches is that the purpose of using a Voice VLAN is that packets will 
be automatically be marked (DSCP cos or BOTH) automatically based on passing in or out of the VLAN.   
That the switch itself will not really prioritize the high priority traffic but it will mark the traffic as a certain priority level  
And other equipment outside of the switch will do the actual traffic shaping..  
Or am I wrong? Do these switches have the capability of actually DOING the traffic shaping?  

Thanks!! 

 

[Martin L]

Maybe on those u mentioned but not on any traditional Catalyst like 2900s, 3500s, 3600s, 3750s, 3850s. Normally you would turn on QoS for voice traffic either auto or manually apply configs per port. Normally, those voice ports are not automatically marked. There are auto commands that you could issue to apply voice QoS for such traffic.
choose a port, add switchport voice vlan x and re-run sh run interface x/y to see if there are additional commands.

Regards, ML
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[balaji.bandi]

Personally - QoS on the all cisco platform are same, only difference is where you deploying this models

SG / RV are small medium - unlike Cat switchswes deploy enterprise Lan depends on cost model of business.

you many not have full control on CLI, the provision can be done using GUI based on the models

[Joseph W. Doherty]

"How is a "Voice VLAN" any different that any other VLAN?"

From a technical perspective in most ways a Voice VLAN is just another VLAN. The only thing that might make such a VLAN a bit special, some switches allow you to define a Voice VLAN in addition to the "normal" VLAN assigned to an access port. Effectively you have a "special" trunk port, where the "native" VLAN is generally the data VLAN and the one, and only, tagged VLAN is the "voice" VLAN.

The reason for having a voice VLAN is to provide additional "protection" to voice hosts on the VLAN. Yes, a separate VLAN makes it easier to manage "security", or QoS, or whatever, but as voice hosts are especially sensitive to "interruption" by traffic they don't want, it's mainly to have voice ports only deal with the traffic they really need.

As to what a switch might, or might not, be able to "do" to a voice VLAN port depends on the features of that particular switch. Again, however, the real goal to try to restrict voice ports to only the traffic they need, nothing else.

[michiganbroadband]

Thanks for the responses!   

I've always just manually created a separate VLAN and put all of my voice equipment on that VLAN and managed it manually.  
I'm also under the assumptuon that using a voice VLAN does not give any kind of priority to voice traffic (over other VLAN traffic) but only "marks" packets as a certain priority (qos).   
If my assumption is wrong I would like to know.  
And yes I understand some (or all)of this depends on the feature set of the switch.  
Imagine the scenario where you have a VLAN trunk to a remote location and that connection is limited to 10mbps FD speed total.  

Running several VLANs across the link as well as a voice VLAN.  
Picture the likely scenario where the 10mbps link can frequently become saturated to 100% with file transfer traffic and the like.   
Would the switch (sg200 or sg250 class) have any built in ability or functionality to drop packets in favor of making sure the
voice vlan packets go through at higher priority than any other type of traffic?  
I assume and imagine that other means of accomplishing this type of task would be required.  
-not anything that is built into this class of switch or most switches that you would come across in this class.   

Thanks!  

[Joseph W. Doherty]

Generally, by default, switches (routers too) don't treat a voice VLAN any different from any other VLAN, except as earlier noted, you can sometimes "define" a voice VLAN as an "additional" VLAN on an access port. Any special treatment generally requires "extra" configuration effort on your part. Some switches, though, have an "easy" way to enable extra "special" treatment via a single command. For example, on some Cisco switches there's an "auto-QoS" command (early/first versions of this command just did "special" treatment for voice traffic, later versions of this command configure much more QoS).

As far as dealing with congestion issues on a shared link, again, many switches support some QoS features that can be used to insure voice traffic receives the treatment it needs to guarantee it will work well. Also again, by default, these features often need to be enabled.

BTW, often misunderstood, network device, again often, by default, don't mark or tag "special" traffic, and also again, by default, don't provide already marked or tagged traffic "special" treatment. Further, you don't need marks or tags on the traffic to treat it "special". (NB: marking or tagging traffic is for device efficiency, i.e. each device doesn't have to analyze a packet, beyond its ToS marking/tag, to determine how the traffic should be treated.)