QoS /VLAN's basics using Cisco devices?

Posted on 2008-11-13
Last Modified: 2012-06-21
I do not know much about QoS and how it works. Our company is growing and we are in phase of implementing Wireless which in turn will support both VoIP and data. Currently we running VoIP in our environment but there are no VLAN's or QoS configured at all. Everything runs on default VLAN. In order to plan and design our new netowork i have few questions:-
// Lets say we have a cisco 2960 layer 2 switches spread all over and a Cisco ___ layer 3 switch as our core switch//
1. QoS:- Does the priority take place at the core layer 3 switch or the layer 2. Say if a packet enters a 2960 switch will it first go to the layer 3 switch and then gets prirotised or does it gets pririotised as soon as it enters the 2960. And how does this happen?  (please do not refer to cisco pages because to understand QoS there is too hard for me)
2. QoS:- If 2960's can do QoS on there own why will we need to get a layer 3 switch?
3. QoS:- Prioritisation take place on what basis ? Is it protocols or ports ?
4. QoS:- If i enable "auto qos voip" on the ports connected to the phone and "auto qos trunk" on my trunk links, does it mean my QoS is running in full and i dont need to fiddle with anything else?
5.QoS:- "auto qos discovery" which switches support it and please please suggest a layer 3 switch which will suite my environment.
6.VLAN's:- Finally, will it be worth to put the VLAN's in place even if you are running the QoS. I know they are totally different concepts but ours is not a very big environment say about 100 computers and about 50 phones.

I know too many question's :-) ,any answers will be highly appreciated. Thanks!!
Question by:WannabeNerd
    LVL 4

    Assisted Solution


    1.  Question:   QoS:- Does the priority take place at the core layer 3 switch or the layer 2.
         Answer:  This depends on which switch the packet enters on...has nothing to do with anything other
                         than layer 1

    2.  Question:  If 2960's can do QoS on there own why will we need to get a layer 3 switch?
          Answer:  You only need a layer 3 switch as a matter of convenience as laye r3 switches do routing
                           and thus is capable of more than a 2960.

    3.  Question:  ...about QoS
         Anwer:  QoS is a vast topic, but I suspect that network performance will greatly be enhanced once
                       you've split your DATA and VOICE packets.  Also QoS only really works when you have
                       multiple sites and speeds thru the telephone company (ISP) can be guaranteed

    4.  Question:  "...enabling Auto-QoS"
         Answer:  Auto-QoS is a fair start, but granular control will not be gotten unless turned off and
                         running a QoS method such as Class-Based QoS.  This will allow you to define which
                         applications/ip's/protocols...etc should be given what amounts of your bandwidth
    5.  See answer 4

    6.  Question:  " Finally, will it be worth to put the VLAN's in place even if you are running the .."
          Answer:  YES, YES, YES, YES, YES!!!!!!  This will do more for you than QoS.  Sounds unreal, but
                           it is definitely true.  Your data packets and voice packets are currently colliding, thus
                           giving you echoing, jitter, low volume, disconnect issues among more.  Split these types.

    hope this gets us started,


    Author Comment

    Q1.If you read the question i have desribed the scenario as well. Packet is entering a layer 2 switch and then we have a layer 3 switch as our gateway. So where will it gets prioritised?
    Q3. is it based on protocols or ports.?
    Q5. "auto qos discovery" command which switches support them and please suggest a layer 3 switch for my environment.????

    If you can clear my conception about question number 1 i guess i will understand most of it (hopefully) :-(
    LVL 18

    Accepted Solution

    qos really comes into play when you go across wan links, so your concerns about layer2 vs layer3 for qos at a single site really do not matter. voip can use a variety of protocols and a range of ports to provide call-control and audio streams and these can be used to classify the audio streams if they do not appropriately classify themselves.
    I agreee with icanhelp that you should not have a single vlan for your lan, it would be better to move off of vlan 1 and leave that for inter switch network communication and put your data on its own vlan, put voip on its own vlan and consider moving other things onto their own vlans as well. like the managment of your network equipment and the network that your servers reside on.

    QoS is not a very complicated topic but is can become pretty sophisticated in its implimentation. I think you would be better served spending your time segmenting your traffic and relying on the autoQoS provided by cisco switchports.

    hope this helps,


    Author Comment

    Thanks decoleur!!
     So what you are suggesting is enable auto qos on switchports and segement my traffic using Vlan's + not worry about implementing QoS. Right?

    Can you please suggest which layer 3 switch i should get  because i will still have to perform inter vlan switching. I also want it to have NBAR on it because it will give me auto qos discovery option.
    LVL 18

    Assisted Solution

    yep, enable qos on the switchports that are conencted to phones and computers but not on non voice related ports like servers and trunks.

    if you are only going to be routing between vlans interfaces on one switch most layer 2 switch can handle that because you are routing between locally known interfaces. a layer3 switch would help you if you had two different routes to the same remote location and you needed something to determine the best path. in short if you are going with a cisco switch a 2960 with the lan base image on it or a 3560 with the standard image on it will do for you.
    for nbar you need to be using a router or a chassis switch, nice feature but expensive to impliment if you do not already have the hardware.


    Author Comment

    I know this is not the part of my actual question but i would be really grateful if you can guide me here.
    Consider. L2 switch connected to L3 switch with trunking working on it.
    On a L2 switch lets say i have 3 vlans
    Default vlan ; port 0/1 assigned.(Management)
    vlan 10: ; ports fa 0/2 - 0/6 assigned to it
    vlan 20;; ports fa 0/7 - 0/11 assigned to it
    and port 0/12 - Set up as the trunk port

    On the layer 3 switch i want to  set up routes in such a way that 10.0.10.x is able to talk to 10.0.20.x .
    On L3 swtich for the interface vlan 10 i assign ip and interface vlan 20 = and set up a route between them.
    1. What default gateway i am supposed to set up on my layer 2 swtich. Will it be or ? Because you can only set up one default gateway. Right? My guess is either one of them. Does it make any difference?
    2.If i ping from a device on vlan 10 to device on vlan 20. There is a broadcast sent to all ports connected that are on vlan 10 and the trunk port and nothing on vlan 20. When it reaches L3 it then routes it to the device on vlan 20.But is it a unicast or broadcast.?

    i will highly appreciate if you can clear it up for me please!!!
    LVL 18

    Expert Comment

    1 the default route should correspond with the subnet that the managment interface on the switch is on. if it ip is on vlan 10 then its gateway should be the vlan 10 layer3 interface.
    2. not quote sure but i think the layer3 interface will do a proxy arp and respond with the mac address of the target on the other vlan if it knows what it is.

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