Simplest way to prioritize VoIP / VLAN

Posted on 2008-11-03
Medium Priority
Last Modified: 2008-11-10

Our company spans over a three floor building. Each floor have the following equipment:
2610PWR-48 for phones
2900-48 for computers.
A total of 6 switches

The 2900's are interconnected via 10Gbps fibre link between the floors. And the 2610's connects to the 2900's in each floor via 2Gbps trunk (2 ports used). To bring the traffic to the Phone Central (connected to the 2610) in the upper floor, a separate VLAN is established (ID2) for VoIP. As an example, VoIP traffic from first floor goes like this:
Phone - 2610 - (2Gbps link) - 2900 1st floor - (10Gbps link) - 2900 3rd floor - (2Gbps) - 2610 - Phone Central.

My question is simple: To have each switch prioritize VoIP traffic, is it enough to go to Configuration tab - Quality of Service - VLAN Priority and set the 802.1p setting for VLAN ID 2 to "7 - Highest Priority"?
Can someone explain if the DSCP setting must be used?
My entry on this page looks like this:
VLAN Name (Vlan ID)          DSCP Policy      802.1p Priority
Telephony(2)                       Disabled            7 - High Priority

And must this setting be made on each switch?
Any comments appreciated.
Question by:riegsa
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Expert Comment

ID: 22866323
Hi. I have no experience with the specified devices. But based on yur description I can say this: You have the VoIP devices on a separate VLAN. So, giving the specified VLAN high priority should be sufficient. With DSCP it is possible to have QoS in an environment where you don't have separate VLAN's. Your VoIP devices send a ToS-bit on all voice-data-packets. DSCP will prioritize the datapackets based on the ToS-bit. So enabling QoS based on both VLAN and DSCP would be double. Setting both VLAN's to DSCP-policy would work too, because the packets on your VoIP-VLAN will all have the ToS-bit. More info here: http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

Author Comment

ID: 22866696
So as I understand you, this setting is correct. We give the VoIP VLAN the highest priority in 802.1p and leave the DSCP disabled. But for this to have any effect, I assume we must use the same setting (set it) on each switch (all six) ?

Expert Comment

ID: 22866933
Yes indeed. Every switch has its own QoS. So you should give the VoIP-VLAN high priority on every switch (either by DSCP or by VLAN). Be sure to check all routes that the VoIP travels to your phone-central. Because QoS depends on the weakest link. If there is one connection on the route that does not have proper QoS, everything fails. So you should have QoS on your LAN and if you have VoIP from an internal phone-central to an external phone-central, you must ensure that you have a guaranteed bandwidth for VoIP. If you have Business DSL for example and you have 10 Mbit, it is very well possible that you have an overbooking of 10/1. This would mean that you have only 1 Mbit guaranteed by your ISP. Voice-data takes approximately 32Kb up and 32Kb down. 1 Mb / 32 Kb is max 31 concurrent telephone-calls. If you are on the edge of your bandwidth you should probably verify with your ISP, and see if they can also do QoS based on DSCP (ToS-bit) to make sure your telephones keep ringing.
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Author Comment

ID: 22874192
If the VoIP traffic is prioritized between the switches, it's sufficient because the phone central is not connected to any WAN/DSL link. It has a separate line from the phone operator.

I assume the other settings at the QoS VLAN Priority page should be left at "No override" then?

Accepted Solution

Heathcliff74 earned 2000 total points
ID: 22874268
Yes. The overrides can be used to manually configure the priority levels for different dscp values. But you are not using dscp right now. So the 'high priority' setting for your telephony-vlan is sufficient. Even when you would use dscp, the default values usually don't need any overrides to work properly. It looks to me you are all set to go. You could do a final test with very high network load (start a few very large file-transfers between different machines). Then try to stress the VoIP by initiating multiple telephone calls (internal calls and external calls) and verify if the quality of your VoIP is okay.

Expert Comment

ID: 22918282
Hi, did everything work out? How about assigning some points? Thanks.

Author Comment

ID: 22920349
I don't know yet, because I have not received any feedback from the people working with the telephony system. I'll post back as soon as I know more. But I think you have earned your points, of course.

The problem causing me to post this, is that some users in the building some (seldom) times looses connection when talking on the phone.
I think my next step will be to upgrade firmware on each device and if no solution, contact HP and have som advice regarding network logging. It should be sufficient bandwidth between the floord, though. The telephony connection out of the house is 2Mbps.

Any comments to my strategy is appreciated.

Expert Comment

ID: 22920736
When you would have a problem with QoS, that would probably result in bad quality phone-call, before any line-drops are experienced, however there is no guarantee on that ofcource. If you experience line-drops I would indeed suggest firmware upgrades, because it looks like a software problem to me. We've had almost exactly the same problem here. We had a Draytek router through which all our VoIP calls were routed. We also had random drops and unavailability of some telephones. We did some logging and it seemed that our router sometimes messed-up its internal routing-table. Upgrading to the newest firmware didn't help. We threw the router out of the window and replaced it with a Cisco 1841 and all was well again. Never Draytek for us anymore.

The logging was done on one of our switches. It was a 3com switch. We enabled a monitor session on one of the ports. We hooked a laptop on that port and we ran the tool WireShark: http://www.wireshark.org/ It showed us that for no apparent reason the router stopped passing VoIP-packet on certain connections. There was not even a TCP_FIN packet or anything to close the connection. That made us suspect the router and we were right. Hope this helps you find the problem. If you have any questions, please let me know.

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