From: Pavel Bykov (slidersv@gmail.com)
Date: Thu Dec 04 2008 - 22:49:42 ARST
It's not like that, really.
by default, class-default will not get any reservation whatsoever.
Great test to proove this is create the following policy map:
policy-map TEST
class SOMECLASS
bandwidth percent 5
now apply this service policy on output, and flood the interface with
traffic that belongs to this class map (really flood - reduce the interface
speed if you need to, e.g. slow traffic generator)
By "old logic" or what was said before about all that
"max-reserved-bandwidth", class-default should have gotten 95% of the
bandwidth, or "the rest".
But in reality? In reality it will get... ZERO, ZILCH, NADA, NOTHING. If you
try to ping now in class default, you will have drop rate of something like
99.9% (so it's almost nothing) It of course has to do with the logic of
CBWFQ algorithm which is not published but was tested to the point that the
algorithm is well understood.
That led to very unpleasant starvations in practice. So best practice is to
use max-reserved-bandwidth 100 (which is default new IOSes I believe) and if
you need to "reserve" then do reserve using bandwidth/priority.
On Thu, Dec 4, 2008 at 11:21 PM, <mihai.grigore@onlinehome.de> wrote:
> Dear fellow experts,
>
> I am now reading Wendel Odom's great QOS - Exam Certification Guide book
> where
> he wrote:
>
> "class-default automatically gets 25 percent of the bandwidth" on page 302.
>
> Is this (still) true ?
> Is this the explanation for the default max-reserved-bandwidth of 75% ?
> If so, what happens with the class-default when I configure
> "max-reserved-bandwidth 100" ?
>
> TIA, MIhai
>
>
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-- Pavel Bykov ---------------- Don't forget to help stopping the braindumps, use of which reduces value of your certifications. Sign the petition at http://www.stopbraindumps.com/Blogs and organic groups at http://www.ccie.net
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