From: Scott Morris (smorris@internetworkexpert.com)
Date: Fri Dec 05 2008 - 12:43:45 ARST
You're talking about two different things...
Bandwidth commands = guarantees in case of congestion.
75% rule was designed to give SOME bandwidth (unguaranteed) to the "other
stuff" on a link.
Now, there's nothing against you making the math work against you. I know
plenty of people who work with over-subscribed lines. And they plan QoS
based on a higher-than-is-really-there bandwidth.
You can trick the router all you like (your "unpleasant starvations") but
that doesn't make it MQC's fault! Honestly, if your congestion is bad
enough to drop the througput that much (more than 25% of link speed) you
have issues. If you have oversubscribed it to the point where reality
intervenes, you have issues. If you allow 100% reservation, and try to
"guarantee" that much, if the transmission speed either actually drops, or
congestion is occurring where packets get dropped, you have accomplished
nothing by "guaranteeing" bandwidth.
You may see more output drops. The point was never to "guarantee" stuff for
class-default. It was to help prevent people from creating their own bad
scenarios. Unfortunately, when we think we are smarter than the routers, we
often find ways to create the potential for bad stuff no matter how many
safeguards were put in place!
Just my thoughts...
Scott Morris, CCIE4 #4713, JNCIE-M #153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
Senior CCIE Instructor
smorris@internetworkexpert.com
Knowledge is power.
Power corrupts.
Study hard and be Eeeeviiiil......
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Pavel Bykov
Sent: Thursday, December 04, 2008 7:50 PM
To: mihai.grigore@onlinehome.de
Cc: ccielab@groupstudy.com
Subject: Re: class-default is reserved 25% of the configured BW ???
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
>
>
> Blogs and organic groups at http://www.ccie.net
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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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