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VoIP Phone over VLANs

Hi guys:

Just asking a couple of questions:

I will be setting my network with vlans using 3560-PoE switches and ill be using IP-Phones + Desktop per interface connection and my current IP settings are in Class B IP address (

My IP Phones are in /16 ip block and my Desktops are in /16 ip blocks of course they will communicate because they're in the same SubnetMask.


If i make vlans(eg;below), will ip phones still works if i change the IP Phone mask to class C /24.?

(VLAN10)Desktop Group 1: /24

(VLAN20)Desktop Group 2: /24

(VLAN30)Desktop Group 3: /24


If I assign the IP-Phone IP block to a certain VLAN, how will the addressing be considering Desktop and IP-Phone are in different IP block using a single interface?


Can I set a single Interface to accept multiple vlan membership just to make sure the two different device can use their IPs without VLAN conflicts?


Re: VoIP Phone over VLANs

The answer to all your questions is, Yes. As you suggested, you can create smaller subnets to meet this requirement. A physical switchport can support data & voice VLANs simultaneously. There are two ways this can be done. See the below link for more details.

As a matter of fact, you are better off keeping the voice and data traffic separate to avoid problems with voice quality. Processing in voice VLAN should be kept to a minimum for better performance. Keeping data/voice in separate VLANs would limit the broadcasts in voice VLAN as that would be a preffered setup for voice setup. Moreover, any STP problems in your data network will not impact the voice VLAN.



Re: VoIP Phone over VLANs

Q1: It is good design practice to use different vlans for voice and data. The sizing of these may very with the number of devices that is required. Do not make them too large (>250 devices), it will reduce performance.

Q2: IP adresses and vlans are not necessarily related. They are on different layer of the OSI model. The connectivity between data and IP phones must be realized at layer3 via IP routing. On a network we most often assign a certain IP range to a certain vlan. Two vlans are not interconnected; we could just as well use two hubs to assign the ip's to.

Q3: A switch port that's configured as a trunk does exactly that. For the ports where an IP phone is plugged in the basic config is as follows:

int fa0/1

switchport access vlan10

switchport voice vlan 11 (for example)




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