Questions regarding the QOS

Unanswered Question
Nov 26th, 2007


I recently took my ONT exam.

But i still have confusions on some questions

1. How scheduler works between classes in CBWFQ if there is no LLQ?

2.ONT guide says that there is only one LLQ can be configured in one policy-map. But QOS exam rejects this theory. I managed to create 2 LLQ in one policy-map. So how scheduler works in this case? Which LLQ gets first priority? FIFO?

3.As we know default queuing strategy on Ethernet interfaces are FIFO.This is hardware queuing. Even when we put WFQ in Ethernet interface we just enable software queuing and nothing changes with hardware queuing. It still remains FIFO is it right? The Serial interfaces have by default wfq. Does it mean software queuing only? My understanding remain the same that hardware queuing is always FIFO, whatever interface we are talking about.

4. From QOS study guide.

CBWFQ says that only class-default can be WFQ or FIFO. Other classes uses FIFO strategy! But when we configure CBWFQ on serial interfaces(WFQ default), the "show policy-map interface xxxx" command shows WFQ queuing in each class! Is it Bug?



I have this problem too.
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bvsnarayana03 Tue, 11/27/2007 - 07:35

Hi Leo,

Looks like you have clearly understood the concepts, but have a little doubt. Hope by the end of this explanation all your doubts would be clarified.

1. I've gone through a lot of explanations on CBWFQ, finally I came across this in Wendell Odom's QoS guide, "Scedhuling Algorithm for CBWFQ is not published by cisco". But the result of the scheduler provides a percentage guaranteed BW to each queue as configured in each class.

2. Yes you can have multiple LLQ's within a policy-map. Here you are absolutely right, between LLQ's FIFO logic is used. You cant reorder packets between diff LLQ's.

3. Again you are right here.The interface always process packets as FIFO (hardware queuing). When interface is busy pushing packets, a large number of packets are queued in memory & the queuing methods are defined to effieciently handle these queues in memory (software queuing).

4. CBWFQ can have 64 queues. For the 63 queues, Within a single queue packets are processed FIFO. For default class, packets can be either FIFO or WFQ processed. Default is WFQ.

pls rate if helped.

mohindersingh Tue, 11/27/2007 - 09:41

Hi Leo,

Here is my answers to your query..

1.) If there is no priority queue so all the classes will be configured with weighted fair queuing and traffic will be served based on the bandwidth allocated to the class.

2.) We can configure more than one priority queue in a policy map. for ex:- if you configure two priority queues, traffic will be served with a single queue with different flows and each flow will get the bandwidth accrding to priority command.

3.) There are two types of queing a.) hardware queuing b.) software queuing

Hardware queuing is the queue of the hardware.for ex:- router and switch interface queue.

software queue will come in picture with QOS. when we apply Qos, first the packet will go in a software queue and then it came to hardware queue of the router or switch.

so if ethernet interface hardware queue is FIFO, it ll remain FIFo only after applying QOS also. it means the queue which are seeing is the hardware queue which will be same throughout.

4.) CBWFQ apply WFQ on all the interfaces and you can apply WFQ or FIFO any queuing methods according to your choice on default class.



bvsnarayana03 Tue, 11/27/2007 - 21:14

Hi Mohinder,

I tried a lot to find how scheduler works in CBWFQ, if you have found any document pls share with us for learning. Because I'm referring Odom's QoS guide & all answers have been taken from that reference.

Howz KC n/w team doing?

Leo_Stobbe Wed, 11/28/2007 - 00:46

No satisfied yet with 1st question..:)

1. In CBWFQ traffic serviced with WFQ inside of each class...But i asked about scheduler between the classes.

bvsnarayana03 Wed, 11/28/2007 - 00:54

dude, cisco has kept it a secret. It has not published the algorithm yet.

pls rate all helpful posts.


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