Re: [cisco-ttl] qos ne zaman devreye girer

From: Devrim Yener KUCUK <dkucuk_at_....>
Date: Mon Mar 27 2006 - 11:52:41 EEST


Selam arkadaslas

Bu konu ile ilgili cok yanlis anlama oldugu icin detayli bir aciklama gondermek istedim

Ornegin %75 kavrami FR icin dogru degil. FR konfigure edilmisse sadece minCIR'e bakilir Eger minCIR yoksa, CIR/2 degeri QoS icin kullanilir. %75 kavrami diger interface tipleri icin gecerlidir ( ATM, ethernet...) Yani frame-relay interface'de QoS icin hangi deger kullanilir diyorsaniz, bu deger oncelikle minCIR degeridir...

Ayrica QoS sadece congestion olunca devreye girer. Yani congestion olmayinca "priority" yada "bandwidth" ne kullanilirsaniz kullanin, kullanilmayan bandwidth diger class'lar tarafindan paylasilir.

Ayrica bu tur QoS uygulamalarinda mutlaka trafik yapisini bilmeniz gerekir.

Bu konu ile ilgili birkac lab testi yapmistim. Onu sizinle paylasayim. Ingilizce ama cok basit bir anlatimi oldugu icin gonderiyorum.

Kolay gelsin

Devrim

To keep in mind in CBWFQ in Frame-relay

When the bandwidth and priority commands calculate the total amount of bandwidth available on an entity, the following guidelines are invoked when the entity is a shaped Frame Relay permanent virtual circuit (PVC):

-If a Minimum Acceptable Committed Information Rate (minCIR) is not configured, the CIR is divided by two.
-If a minCIR is configured, the minCIR setting is used in the calculation. ( this means the available bandwidth that will be used for the QoS will mincir value)
-Avoid setting the CIR or minCIR at the access rate. Otherwise, you may see output queues building up and causing big delays in CBWFQ classes. The reason is that the shape rate does not take into account the overhead bytes of the flag and Cyclic Redundancy Check (CRC) fields, so shaping at line rate is actually oversubscribing and will cause interface congestion. There really is no reason to shape at the access rate. You should always traffic shape at 95 percent of the access rate or, more generally, the aggregate shaped rate should always be 95 percent below the access rate.

For the bandwidth usage:
http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080 103eae.shtml#howisunusedbandwidthallocated

"it is important to understand that since the priority classes are policed during congestion conditions, they are not allocated any remaining bandwidth from the bandwidth classes. Thus, remaining bandwidth is shared by all bandwidth classes and class-default. "

Let us understand the difference of ATM and Frame-relay in CBWFQ: 100 percent, is 75 percent of interface BW in ATM or ethernet. So for ATM, only 75 percent BW is visible by bandwith and priority command. This means even if you DONT define a "default-class" the rest of the traffic will pass during congestion
For Frame-relay this is NOT true....
The visible BW for FR is minCIR (for all class include default-class). To quarantee bandwidth for default-class, bandwidth statement should be put explicitly under class-default configuration. This means during congestion if we DONT want any other traffic, we should explicitly define this, like by permitting any for the default class.

And the usage of excess bandwidth:
If excess bandwidth is available, the excess bandwidth is divided amongst the traffic classes in proportion to their configured bandwidths. If not all of the bandwidth is allocated, the remaining bandwidth is proportionally allocated among the classes, based on their configured bandwidth."

Let's look at two examples.

In the first example, policy-map foo guarantees 30 percent of the bandwidth to class bar and 60 percent of the bandwidth to class baz.

policy-map foo
  class bar
    bandwidth percent 30
 class baz
   bandwidth percent 60
Applying this policy to a 1 Mbps link means 300 kbps is guaranteed to class bar, and 600 kbps is guaranteed to class baz. Importantly, 100 kbps is leftover for class-default. If class-default DOES NOT need it, the unused 100 kbps is available for use by class bar and class baz. If both classes need the bandwidth, they share it in proportion to the configured rates. In this configuration, the sharing ratio is 30:60 or 1:2.

Second example: ( 4 different classes and 1 class-default) policy-map Canberra-WAN
  class File-Transfer
   bandwidth percent 10
  class 123
   bandwidth percent 10
  class ftp
   bandwidth percent 10
  class 102
   bandwidth percent 10
  class class-default
   bandwidth percent 55 <===

What will happen if we explicity define or NOT define "bandwidth percent 55" in the class-default.
Let us assume we have only 3 data streams and using 3 classes, first second and third.

Answer:
- Without 'bandwidth percent 55' under class-default: the 3 configured classes share the
unused bandwidth of the 4th class amongst themselves and leave nothing to class-default
- With 'bandwidth percent 55' under class-default: the 3 configured classes and
class-default share the unused bandwidth of the 4th class amongst themselves

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Received on Mon Mar 27 20:32:53 2006

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