Why doesn't QoS work the way I want it to? Or does it...

Posted on 2014-08-13
Last Modified: 2014-09-25
Or does it...
At my office there is a Cisco switch (not sure of model) that has QoS configured so that the VoIP phones have high priority. When my PC starts doing cloud backups (Carbonite) it takes up all bandwidth. When we answer a call, the phones "crap out" because there's no bandwidth. Why aren't the phones given priority and my PC knocked down?
Question by:ITworks
    LVL 3

    Assisted Solution

    The problem is not your switch, but your router. The switch is probably honoring the QoS but that ends once you go outside your LAN, either to the WAN or internet.
    LVL 4

    Author Comment

    Is there a way to set QoS on our Cisco router or to get certain types of devices higher priority to the WAN?
    LVL 3

    Accepted Solution

    Yes. But it will end in the next hop unless you have QoS configured on the WAN itself (MPLS?)
    LVL 4

    Assisted Solution

    by:Ruel Tmeizeh
    You need to run some monitoring commands on your switch to determine if it is actually handling QoS marked packets/frames properly. Some popular Cisco switch models (2950 and maybe others) have the QoS defaults set to different values than what most phones are setup to use, so the phones and the switch may both have QoS turned on, but they aren't agreeing on the priority number markings and thus nothing is prioritized (or at least not very well).

    IF your switch and the phones are indeed setup properly, and the switch is correctly queuing frames, then as nickoarg said, it may very well be your router that isn't setup to properly deal with the traffic as it goes out to the internet, thus your QoS is only working on your LAN, and as soon as the router gets any traffic it treats it all equally. It's settings for proper values will need to match what the switch and phones are sending and expecting in order for the setup to work as you want.

    The easiest way to setup QoS is to not worry about "types" of traffic, but to do it based on VLANs. If you have all your phones set on one VLAN and your PCs on another, you can set the switch to mark all packets/frames to and from the PC VLAN as normal priority, and all packets/frames to and from the phones as high priority. Then set your router to either force high priority for the voice VLAN, or trust the priority of the switch and queue based on that, and that's all there is to it. This is the simplest way of doing it. The downside is that if you use softphones on your PC(s) then they will only have normal priority, and softphone call quality may suffer.

    The more complex way is to have the switch (and/or router) analyze all the traffic and put it into different QoS classes based on the type of traffic. (HTTP, FTP, and such would go into a low priority queue, Bittorrent and Carbonite lowest priority/scavenger, SIP signaling from phones would go into a medium-priority queue, RTP audio from phones would go into the expedited forwarding queue, etc. )
    Doing it this way gets complex FAST and is a lot of config to get it right, and may overwhelm a switch or router that doesn't have lots of free processing power.

    Another way is to set your switch and router to TRUST QoS markings on packets/frames that they receive, and put them into the proper queues based on that. You don't want to do it this way in an enterprise network where you can't trust everyone using the network, but otherwise it works great and doesn't require much router or switch processing power. After you have setup your switch and router to trust marked packets and frames, which is a relatively simple config, you must then setup your phones to mark their packets with the proper QoS markings (most phones already do this, but some tweaking of the values may be required). You can leave your PCs as-is, and they will be given normal priority, or if you need to run softphones, you can usually have the softphone app tell your OS to mark the voice packets that are sent out (how this works depends on the OS you are running, whether OSX, Windows, or Linux).

    You didn't give a whole lot of detail as to your exact configuration, but hopefully the info I've given will make it clearer for you how it works and what areas of your network need attention.

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