Explanation on QoS Command and Effect

[Ken Lee]

Hi,

Anyone kind enough to explain this to me?

I have the configuration as below. I just would like to know the effect if let's say for example , class class-default. Current Bandwidth remaining is 25 percent of  2MB which is 0.5MB. So Let's say, there is traffic falls under this class which is more than 0.5MB but there isn't any traffic for video, will the traffic being restricted or it will still go through as there isn't any traffic for video which is 22 percent?

Hope some experts can enlight me on this. Thanks!

policy-map CBQ_2M
class class_Net_Critical
    bandwidth remaining percent 6
    queue-limit 64 packets
class class1-voice
    priority 570
class class1-video
    bandwidth remaining percent 22
    queue-limit 128 packets
class class2-signalling
    bandwidth remaining percent 6
    queue-limit 64 packets
class class2
    bandwidth remaining percent 31
    queue-limit 75 packets
class class4
    bandwidth remaining percent 6
    queue-limit 64 packets
class class-default
    bandwidth remaining percent 25
    queue-limit 95 packets
policy-map shape_2M
class class-default
    shape average 1900000 19000
  service-policy CBQ_2M
!

Rgds,

KA

[sean_evershed]

Hi, according to the Cisco QoS Exam certification guide, the scheduler gives a percentage of bandwidth to each class based on the configured values. If some queues do not need their bandwidth for a short period of time, then the bandwidth is spread across the other classes.

Please remember to rate all posts that are helpful.

[Ken Lee]

Sean,

I will remember to rate the post if it is helpful.. :)

Just want to clarify more, when you say spread, is it mean by percentage as well? Let's say for the scenario above, There is no video traffic, how does it spread to other classes?

Thanks again!

KA

[sean_evershed]

Ken,

According to the exam guide the actual workings of the scheduling algorithm for CBWFQ have not been published.

The only details available is that it provides a guarneteed bandwidth for each queue.

[Eugene Lau]

Hi Ken,

adding to Sean's point - remaining available bandwidth is allocated proportionally except in the case of the priority queue which is policed at the configured rate.

My favourite links for CBWFQ:

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_q_and_a_item09186a00800cdfab.shtml

and

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml

They give pretty good examples.

HTH

Eugene

[Ken Lee]

Eugene,

Thanks for your reply.

Let's say the Policy Queue 670kbps, and only 400kbps occupying the Class Voice, So can I say the balance of the packets 270Kbps would be able to utilise for other traffic or it is already Occupied for only class voice even though at certain time, 400kbps occupying the voice traffic only.

Thanks,

Regards,

KA

[cadet alain]

Hi Ken,

when there is no congestion  the CBWFQ config doesn't apply.

When there is congestion and some traffic are not passing through then other class can use the bandwidth but Cisco doesn't tell how this is performed.

Regards.

Alain.

[Eugene Lau · Accepted solution]

Hi Ken,

I think you're refering to the priority queue set at 670kbs

So if there's 400kpbs of traffic going through priority queue (PQ), the remaining 270kpbs would be shared amongst the other queues.

from the link

"

Q. If a class in Class Based Weighted Fair Queueing (CBWFQ) is not using its bandwidth, can other classes use the bandwidth?

A. Yes. Although the bandwidth guarantees provided by issuing the bandwidth and priority commands have been described with words like "reserved" and "bandwidth to be set aside", neither command implements a true reservation. Meaning, if a traffic cla...

Do note the discussion about the PQ characteristics under congestion.

A great way to see how things behave in the real world is to run a few test. IPERF is a simple way of generating traffic between two PC's

HTH

Eugene

[Ken Lee]

Thanks Eugene!

Have rated your post

[Eugene Lau]

Thanks Ken! Feel like pay day

Take care for now!

Eugene.