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Background Information

Created by dave. Last edited by dave, 16 years and 274 days ago. Viewed 17,822 times. #2
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Background information

This is all background noise copied from >>a thread on the Dell Forums; it is reproduced here because such things have a habit of going missing. I didn't slavishly follow any of this, but it pointed me in interesting directions.

Thread: QOS config for Data and VOIP [ NEW ]


We have 3448P,5324 and 6024F switches in our network and I need to set up QOS for phones that have a computer connected to them,,,I'm not sure where to begin Also does a 6224 do auto QOS

Bill Felling:

I think it depends on how the phones do QOS, but here's a brief synopsis of my experience with Cisco phones.

Out of the box, all of those switches support queues based on 802.11Q (Vlan class of service tagging). They also support Diffserve, but by default I believe they rely on 802.11Q. The number of queues they support varies by switch model - 4 for the 3448 and 8 for the 6024, I believe. And of the four supported on the 3448, I think one is reserved, although I'm not sure for what. But that still leaves enough to separate voice packets.

Recent Cisco phones automatically insert both types of tags into voice packets. So if you

  • set up a vlan for your phones,
  • make sure all ports which are connected to phones are part of that vlan in Tagged mode (so the vlan tags coming from the phones are preserved), and
  • make sure all interconnect ports are also part of that vlan
your voice packets will be placed into the highest priority queue on the switches, and get forwarded ahead of everything else. You will also need to make sure your forwarding algorithm is "strict priority" instead of weighted-round-robin, so that all top priority packets get forwarded before any lower priority packets.

Cisco servers, on the other hand, use Diffserv exclusively, since the servers aren't typically vlan aware. So the switch to which the server(s) is/are connected needs to know to pay attention to diffserv - at least on the ports connected to Cisco phone servers. Finally, if you want to make sure that packets with diffserv priorities are treated exactly the same way as those with class of service tags, you need to modify a couple of the diffserv mapping entries.

But other phone systems/phones may handle this completely differently.


Ok, first off, you will need to create another VLAN. Some phones may have a preset VLAN ID so that is the ID you will use when you create the VLAN, if not, VLAN 2 will be fine. VLAN 1, the default VLAN, can be used for your data and the 2nd VLAN for VoIP traffic. Lets say your VoIP server is on your core switch, that port that it is connected to will need to be configured to be on VLAN 2 only.

console(config)# interface ethernet g1 (port 1 on a 6024 or 5324 switch) console(config-if)# switchport access vlan 2

That will put the port into VLAN 2 only, so all traffic entering that port is in VLAN 2. Now lets say that you have your phones and PC's connected to a 34xx switch. Lets also assume that you have a PC daisy chained to a phone then to a switch. That port will need to be in a trunk mode to trunk both VLANs on that link. VLAN 2 will be tagged while VLAN 1 will be left untagged, it will also be the native VLAN for that port and that is where all untagged traffic will automatically get dumped into. You want your data left untagged since most PC's don't understand tagged packets.

console(config)# interface range ethernet e(1-12) (ports 1 - 12 on a Powerconnect 3424 switch) console(config-if)# switchport mode trunk console(config-if)# switchport trunk allowed vlan add 2

Every port has a native VLAN ID of 1 by default, if you were not using VLAN 1 as your data and let's say you are using VLAN 20, this is the last command you would type.

console(config-if)# switchport trunk native vlan 20

The same configuration will be done regardless if you have a PC connected to a phone or not.

Now for QoS. Your VoIP server should have a setting for QoS. The easiest to use is Diffserv. By default, the Dell switches have a DSCP to Queue mapping all ready setup. If you look in the web gui, go under QoS settings, Global Settins and enable it and also set the Trust Mode to DSCP. Go to DSCP to Queue Mapping and look at the values and the queue they are mapped to. The easiest thing to do is find a value that is queue at a priority of 3 and assign that value to your VoIP. It's pretty much that easy. I would not recommend changing the priorites in the mappings. Usually most VoIP's are set to a value of 46 that has a mapping of 3. System processes use a priority of 4.

That is pretty much it on a basic VoIP setup.

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