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voice and data vlans

I am going to install a cisco 2851 with CME and IP telephones.

The router will be used for data so the PC's and IP telephones will be in the same network.

I want to do voip traffic prioritization, so which is the best option for doing it?

Vlan's? Can I have unique DHCP server for IP telephones and PC's? Is there any good documentation for starting?

Super Bronze

Re: voice and data vlans


You will still want to put the phones and data stuff in seperate VLANs.

If you have an external switch then create a dot1q trunk to the router.

This will make prioritizing voice easier as you can use the Voice VLAN (as well as the DHCP values) to classify traffic.

Also if the phones and data are in seperate VLANs then you can have seperate DHCP servers. Just add the appropriate ip-helper to each dot1q subinterface on your router's LAN port.

Hope this helps


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Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
Community Member

Re: voice and data vlans

Theoretically you may achieve prioritization with or without separate VLANs. QoS may work both, by IP address (or range), or by Protocol/Port. However, from the management perspective, it's better to split two "environments" into the separate VLANs and Subnets.

Now some questions. What switch do you have for your PCs and phones? Is it Cisco switch? If it is, what model?

If you have Layer 3 switch (Catalyst 35xx/36xx/37xx series switch or higher), then you can do VLANs and routing on the switch itself. If it's Layer 2 switch (Catalyst 29xx series. etc), then you may configure the "uplink" port (port, connects to the router) as a 802.1Q trunk, and do routing between VLANs on the router. Here is a link to examples:

If you do not have a separate DHCP server on your network, then you can configure DHCP on the switch or on the router.

Then, you may start working on QoS. Here is a link to a configuration examples:

Good luck,



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