Re: CBWFQ Congestion Allocations

Hi Matt,

This is an interesting question and I will try to answer it. Experts please
correct me if I am wrong.

The way I understand things is:
1)Bandwidth is a minimum guarantee that we can surpass, however will always
guarantee in case of congestion.
2)Priority:is an upper limit (policed) that guarantees a high speed service,
and the upper limit ensures that we don't exceed so that we don't affect
other applications. If we exceed the limit I guess traffic can pass but we
can't ensure excellent service no more. In previous implementations of IOS i
guess this was a strict policer.
3) Class default: unused bandwidth goes to class-default.

Scenario 1 & 2 : I guess they will result in same percentages (since in
Scenario 1 after the priority queue is being given its share (15%) all what
you have remaining is 85% to be split up among the 2 classes.
15%/30%/30% (All @ line rate)
TEST1: is policed to 15%.
TEST2: 42.5%
TEST3: 42.5%
Class-default:0% (there is no unused bandwidth)

Petr's blog post is one of the best references on the subjet (Insights on
CBWFQ) can be found:
http://blog.ine.com/2008/08/17/insights-on-cbwfq/

For User Defined classes:
*Weight(i)=Const*100/Bandwidth_Percent (this is the share of a certain
class)* and since
both of them have the same percent they will have the same weight.

The max-reserved bandwidth affects all the classes, since it is the Pizza
from which all the classes take their slices, i.e. the QOS measurements
apply as per my knowledge to the bandwidth which is the max-reserved
bandwidth and not to the original bandwidth of the interface.

HTH,

On Tue, Sep 21, 2010 at 8:32 PM, Matt Bentley <mattdbentley_at_gmail.com>wrote:

> Hi GS:
>
> Question about the exact behaviors of CBWFQ.
>
> Say you have this policy-map:
>
> class TEST1
> priority percent 15
> class TEST2
> bandwidth percent 30
> class TEST3
> bandwidth percent 30
> class class-default
> fair-queue
> random-detect
>
> So I understand the following:
>
> TEST1 no congestion -> EF treatment for up to 15%, Can send up to line rate
> with normal treatment (assuming none of the other classes are sending)
>
> TEST1 congestion -> LLQ treatment for up to 15%, policed for greater than
> 15%
>
> TEST2 no congestion -> Can Send Up To Line Rate (assuming none of the other
> classes are sending)
>
> TEST2 congestion -> AF treatment for up to 30%, excess ????
>
> TEST3 no congestion -> Can Send Up To Line Rate (assuming none of the other
> classes are sending)
>
> TEST3 congestion -> AF treatment for up to 30%, excess ????
>
> For TEST2/TEST3 congestion scenario, I see this statement from Cisco
>
>
> http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml
>
> If class-default does not need it, the unused bandwidth is available for
> use
> by class TEST1 and class TEST2 (they use different names for their
> classes).
> If both classes need the bandwidth, they share it in proportion to the
> configured rates. In this configuration, the sharing ratio is 30:30 or 1:1
>
>
> Instead of trying to word my questions, if someone could elaborate the
> allocations in the following scenarios, it would be immensely helpful.
>
>
> 1) Scenario#1
> TEST1 is sending traffic at exactly at 100% of line rate
> TEST2 is sending traffic at exactly at 100% of line rate
> TEST3 is sending traffic at exactly at 100% of line rate
> class-default is sending traffic at exactly 100% of line rate
>
> How is the bandwidth going to be allocated between them?
>
> 2) Scenario#2
> TEST1 is sending traffic at exactly at 15% of line rate
> TEST2 is sending traffic at exactly at 42.5% of line rate
> TEST3 is sending traffic at exactly at 42.5% of line rate
> class-default is sending traffic at exactly 50% of line rate
>
> How is the bandwidth going to be allocated between them?
>
> Also, I don't think the max-reserved-bandwidth assists in determining how
> much bandwidth the default class gets when congestion occurs, but not sure?
>
> Thanks very much,
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>
>
>

-- 
KJ
Blogs and organic groups at http://www.ccie.net