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Imran Ahmad

How QoS Bandwidth Calculation works

Hello Guys,

I want  to setup qos rate-limiting.  i want to allocate 50% of my internet link bandwidth to PRIORITY-QUEUE, and 50% of bandwidth for other web-traffic and default-traffic.

My question is :  while i configure and alocate 50% of bandwidth for priority-queue,  is that 50% calculated from the Fastethernet interface speed or it is calculated from my real internet bandwidth which i have taken from ISP ??

To make it clear >  lets say fastethetnet interface speed is 100Mbps, and the bandwidth i have purchased from ISP is  512kbps,  so while configuring QoS and alocating  50% of bandwidth to my Priority-Queue   so does the router  allocats  50% of my fastethernet interface speed (which is 50-Mpbs) to my Pri-Queue or does it allocates   50% of my internet-bandwisth (which is 256-kbps) to my pri-queue ?????

I want to know how the allocation of bandwidth is calculated ?

Please help

Alessio Andreoli

Thanks for the link, it has usefull information.  but it did not reply to my question.

My question is the Internet Assigned Bandwidth (by ISP) and the Router Interface Bandwidth. the percentage we define for a traffic-class, is it deducted from the Interface Bandwidth or the Internet-Bandwidth which is provided through the ISP ??

Hello Imran,

in your case you need to used Hierarchical QoS: The parent policy creates a logical pipe at 512 kbps that fits with the real internet link speed and invokes the child policy that implements queueing


policy-map SHAPE512k

class class-default

shape average 512000

service SCHED

policy-map SCHED

class VOIP

priority 256

class class-default


interface type x/y

service output SHAPE512k

without the nested policies the interface speed is the reference used.

Hope to help


thanks for the information above.    another question : while i want to put my Policy-Map  into my interface input direction i get the following error message " CBWFQ : Can be enabled as an output feature only "  .         

what does it mean?   does it mean that we can apply QoS only at the router outbound direction only ???  if it is so then how to control Downloading on a router.  Downloading is important than uploading, thus controling incoming traffic to a router is much more important than outgoing traffic.

Plz instruct .

One else question, Why i can not  enable FIFO queuing on my Class-Default or any other classes ?  there is no fifo option availble

Hello Imran,

yes queueing is supported only outbound. And CBWFQ is a form of queueing-

You can control what you transmit and you cannot control what you receive.

You would need an agreement with ISP to implement QoS on the other direction.

the FIFO queue is used when CBWFQ is not configured, you should be able to configure fair-queue that is a different thing on the class class-default.

Hope to help


So if we can not control what we recieve.  Then what the ( service-policy input ....) interface command is for ?

How can i control downloading or incoming traffic to my router from outside ?  is there any other way rather than making agreement with the ISP ??   I have seen alot of people who can control incoming traffic without asking thier isps for any QoS agreement


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Just to clarify what Giuseppe was noting, you cannot use CBWFQ queuing features, except outbound.  However, you can use some CBWFQ features, inbound, such as marking and policing.

You can somewhat control inbound traffic using policing, but that only rate-limits and it deals with congestion after-the-fact.  I.e. it can be better than nothing, but it's often not nearly as effective as upstream egress congestion management.

Hello Imran,

a service policy command invokes a policy-map.  Inbound are supported only policy maps that perform policing or marking (changing the IP Diffserv bits on the incoming packets).

Policing is a form of rate control that can be performed per traffic class, traffic in excess of the configured rate is dropped.

In outbound direction you can perform the following actions in policy maps

police, mark, shape and queueing

Shaping is different from policing because traffic in excess then the configured rate is buffered (delayed) instead of being directly dropped

So the most efficient way to implement QoS downstream would be to have QoS configured outbound on the SP router and this is why I have written of a need of cooperation with the SP for this traffic direction.

Hope to help


Ok I got it.  there is an idea in my mind : to control outgoing traffic / uploading  we can normally setup QoS at the outbound direction of the Router interface facing the ISP,    and to control Downloading / incoming traffic to our network -if i setup QoS at the outbound direction of my Router's interface (Inside interface) which is connected to my LAN. How you think wont that be a better solution to control incoming traffic from internet ? instead of seting up Policing or making any cooperation with the ISP ? 

How it works i dont know, but that is what i think. plz tell me wat u think?

CBWFQ means Class Based Weighted Fair Queue.

To answer your latest question, Imran: If you put the QoS policy outbound on your router's inside interface, it won't have any effect on your WAN link. The 'default' traffic might still be sent inexcess costing you your voice bandwidth and will only be dropped once it reaches your router. So it won't have the desired effect. So basically for effectively implementing QoS inbound you'd need to work together with your ISP.

Config T

The best way to implement what you're trying to do is by working with your ISP. If that is not possible, then apply an outbound shaping policy on your inside interface.