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VLAN and VOIP traffic

Posted on 2014-01-28
Medium Priority
Last Modified: 2014-02-05

I am trying to work out VOIP quality problem in one of our remote locations.  Users complain of calls being choppy or the users cannot hear the party calling them.  I am sure I need the segment my network or enable some sort of QOS, but I am having problems working out what needs to be done exactly in order to resolve this problem.  

Currently, we have a Trixbox server that has POTS lines plugged into it on our LAN.  We then have Aastra phones in each office and then the computer is plugged into the phone.  In some instances there is a switch in the employees office and then the phone and computer are plugged into the switch since I only have one line that runs from the server room to the employee's office.

Network Infrastructure:

Primary Internet             Backup Internet
                         ASA 5505
             Netgear   GS748TPS Switch
           Computers, Phones, Switches

I can't use the VOICE Vlan on the switch since the computers are plugged into the phones and I don't think I can use another VLAN because that would cause the phone to have another IP Address and thus would cause the computer not to be able to access the network.  

Can someone please tell me what can be done in this network configuration to properly segregate my traffic so that I can improve voip call quality on the network?

Question by:krhoades7601
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LVL 17

Expert Comment

ID: 39816569
Replace the ASA5505 with an Adtran router, more expensive but you need the QOS functions as well it allows any configuration needed to connect to your VOIP provider.

Connect the phones to the Adtran to control packets going to the phones, so VOIP doesn't get squeezed out by video, email, etc.

Link the NetGear switch to the Adtran and run the workstations off this switch.
LVL 12

Expert Comment

ID: 39819517
I can't use the VOICE Vlan on the switch since the computers are plugged into the phones and I don't think I can use another VLAN because that would cause the phone to have another IP Address and thus would cause the computer not to be able to access the network.  

This is not true.

You can create vlan for voice and assign that vlan to switch interface as voice vlan.

The phone will receive IP subnet from voice VLAN and PC will receive IP subnet from Data VLAN.  You will need to add DHCP scope on your server for the new VLAN subnet.

If your switch is layer2, then your firewall should do the intervlan routing if needed.

Accepted Solution

Ruel Tmeizeh earned 2000 total points
ID: 39821975
It's possible you have two separate issues here: the choppy voice (lack of QoS) may be separate from the one-way audio.
But for sure, properly segmenting your network and enabling QoS should fix the choppy audio. What handles DHCP for the network? The ASA?

Here is what I recommend: Completely separate your voice and data networks logically. The voice VLAN is there for a reason---use it! :-) It sounds like a big deal, but it's well worth it, and actually makes for a simpler network design.

To clarify:
I'm assuming that the ASA is the DHCP server on your network, if you use the Trixbox server, then you would do this a bit differently, but the overall concept is the same.

1. Create VLAN 2 (or whatever VLAN you'd like to use for voice) on the switch, and set it up as the Voice VLAN.

2. Assign the voice VLAN to all ports that have phones on them.

3. Set the port that the Trixbox server is on to VLAN 2.

4. Create a sub-interface on the ASA for VLAN 2.

5. Setup another DHCP scope/pool on your ASA for the VLAN 2 clients.
If you are required to use certain address ranges and subnets in your organization, then you may have to do some subnetting to split your current subnet into two parts, or use a designated range or whatever. If not, then just choose an unused class C block like

6. Set your phones to use VLAN 2 (and if possible for security, filter VLAN traffic on the PC port).

6. Test to make sure everything is working. It will probably perform the same at this point, since there is still no QoS, but make sure that all the phones and PCs are properly able to communicate and get their IP addresses and such.

7. Setup QoS on the switch. Since you have analog FXO lines and are not using a VoIP provider, and no voice traffic will be passing through your router (the ASA), you can setup QoS either at layer 2 using 802.1p, or at layer 3, using DSCP markings. If you had VoIP traffic going through the router, then you would need to use DSCP, so you probably want to go ahead and use DSCP to be ready should you ever need external VoIP.

In the switch's QoS settings, you will set it to respect/trust the QoS markings that the phones and the Trixbox server will provide by mapping the high priority DSCP markings to the highest forwarding priority queue.

8. Set the same VLAN and QoS settings on your other switches in the other offices.

9. Make sure your phones are set to mark packets with DSCP markings.

10. Set your Trixbox server to mark packets going in/out with DSCP markings. You do this in the sip.conf file (or the appropriate file that FreePBX wants you to use) by adding these lines inside the [general] section:


This gives SIP signaling packets moderately high priority, RTP video packets high priority, and RTP audio packets expedited forwarding, which is the highest priority.

11. (optional) Configure the ASA to respect DSCP markings on traffic as well, which will affect traffic going through the ASA out to the internet and vice versa.

That should be it. It takes a bit extra work to set all that up, but once you do, adding phones and PCs in the future should be a snap, and should have no performance impact on your voice traffic.

Author Comment

ID: 39836972

Thank you so much for the detailed information.

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