In the following scenario, is it possible to prioritize VOIP/RTP from x.x.x.51 through the gateway with a Cisco switch, and how? I need to prevent x.x.x.50 from saturating the WAN connection.

                    Public Internet
                  Gateway/Edge Router
           (WAN/10Mbps symmetrical connection to ISP)
                    Switch port 1
Switch port 2                               Switch port 3
  |                                           |
  |                                           |
x.x.x.50                                   x.x.x.51
Firewall/NAT                               Firewall/NAT
  |                                           |
  |                                           |
LAN/Desktops                               LAN/Phone System  

Do I need to tell the switch to limit the speed of port 1 to the 10/10Mbps of the gateway/WAN?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 13 '17 at 2:56

QoS is all about fairness, as you define it. There are multiple parts to QoS.

There is layer-2 QoS, which uses 802.1P to set the COS bits in layer-2 headers for layer-2 domains. Cisco switches have full COS marking, but limited layer-2 queuing.

There is layer-3 QoS, which uses the TOS bits in IP packets. Cisco routers have pretty extensive layer-3 queuing capabilities.

You also have things related to queuing, e.g. shaping and policing. You will need to decide on policies for these.

First you need to classify the traffic. Basically marking the packets with separate TOS/DSCP marking for the different traffic types. This is best done as close to the traffic source as possible, and Cisco switches can do this as traffic enters the switch from the host.

Once you have classified and marked the traffic, you can apply your fairness doctrine. In your case, it sounds like you want a priority queue on the WAN interface for the VoIP traffic. You will need to decide which percentage of the bandwidth to dedicate to VoIP, and configure your queuing accordingly.

Understand that your QoS markings and policies will not be honored on the Internet, and that you have no real control over incoming traffic, since by the time you see the incoming traffic, the bandwidth has already been used. Since VoIP is a two-way conversation, you can have perfect QoS on your network, but still get crappy VoIP calls because there is no QoS on the Internet, and your incoming WAN link can be saturated, and your QoS policies have no real effect on that.

QoS is a really huge subject, too large for this site. You would need to provide a lot of information about your devices and their configurations, and make some decisions about a lot of things. There is no real way to say, "this" is the answer to how you should accomplish what you want.

  • In my situation, the need for QOS rests solely on keeping moments of high demand on the first LAN from disrupting calls on the second LAN. They get clear audio on their phones all day, until someone, say, starts a 900MB iPad update, then the audio gets choppy. I need to configure the switch to queue traffic that threatens to drop UDP packets from ports 10000 to 20000.
    – medbot
    May 7 '16 at 1:52
  • Implementing QoS requires a comprehensive network-wide set of policies. You need to do some research, and make some decisions.
    – Ron Maupin
    May 7 '16 at 2:34
  • @medbot, by the way, the iPad update, which disrupts your VoIP, is incoming traffic, and I explained that you have no real control of incoming traffic since the WAN bandwidth has already been used by the time you see the traffic on your network. You can block the traffic one it reaches your network, but that will just cause the external source to resend dropped packets, lengthening the time it takes to get the upload (causing a longer VoIP disruption).
    – Ron Maupin
    May 7 '16 at 20:12
  • When you say voice gets choppy, where are the other phones? On the same LAN? Across the WAN? You seem to be missing sone details in your diagram.
    – Ron Trunk
    Jun 6 '16 at 1:02

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