bandwidth allocation | default class|CBWFQ

[sarahr202]

Hi everybody

Let say we have 100 mig circuit. Max -reserved bandwidth is 100 mig as well.  We make following allocations:

Class A

bandwidth 20

Class B

bandwidth 60

Class  Default.

1)We did not make any bandwidth  allocations for default class.  Assuming We are congested ( i.e class A, class B ), What is the maximum bandwidth Class Default can use?

2) Let say we are congested ( classA,classB) but there is no traffic in default class. How will this  unused 20 mig will be distributed among these classA and class B?

++++++++++++++++++++++++++++++++++

I am getting confusing answers:

For example:

From one of theblog ( Dont want pick on author so did not quote it)

You'll want to configure a bandwidth command under the class class-default Otherwise, IOS will divide any unallocated bandwidth equally among all classes; this can result in the class-defaulthaving a very small amount of bandwidth.

Cisco QOS  Documentation says:

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_cr.pdf

From the above link:

The following output from theshow policy-map interfacecommand on serial interface 3/2 shows that 500 kbps of bandwidth is guaranteed for the class

named voice1. The classes named class1 and class2 receive 50 percent and 25 percent of the remaining bandwidth, respectively. Any unallocated bandwidth

is divided proportionally among class1, class2, and any best-effort traffic classes

Which One is true statement?

If Cisco documentation is correct, then what proportion of unallocated bandwidth is given to default class as there is no bandwidth percentage configured under default class.

Thanks

[Joseph W. Doherty · Accepted solution]

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Posting

#2 If there's just traffic for classes A and B, they will proportionally share it 20:60 or 1:3 (or 25%:75%).  (NB: the former is assuming they want "more".  If not, actual usage might not reflect bandwidth allocation.  For example, if class B only used/wanted 50% of link, class A could obtain the other 50%.)

#1 recall finding some blog that really went deep into what class-default gets when you don't explicitly allocate bandwidth.  I also recall it may have been IOS version dependent and whether FIFO or FQ was defined in class-default (this also was pre-HQF).

It was very complicated, and IMO, best avoided by defining bandwidth in class-default, if class-default usage, relative to other defined classes, is important to your QoS policy.

Generally, if something isn't clearly documented as expected behavior, I avoid relying on "discovered" actual behavior, because it might change with next IOS release.