From: paul cosgrove (paul.cosgrove@gmail.com)
Date: Fri Dec 05 2008 - 11:33:49 ARST
Hi Pavel,
I think some documentation might say that class-default had to be
specifically defined and allocated bandwidth in order for it to use any.
Whether or not this was the case previously it is not the behaviour in 12.4
(13r) which I have on my lab kit. With this class-default does not need to
be defined in order to receive bandwidth.
In the example below I have a 100Mbps input port receiving >10Mbps ICMP from
SW2, and some other ICMP from another device a hop away from SW2. The
combined traffic is contending to get out a 10Mbps interface on this router
(fa0/0), but none of the class-default traffic is dropped. I'm using
smaller packets here for the class-default traffic just to illustrate the
point.
interface FastEthernet0/0
ip address 192.168.100.2 255.255.255.0
load-interval 30
duplex auto
speed 10
service-policy output BANDWIDTH
end
Router2(config-pmap-c)#do sh policy-map int fa0/0
FastEthernet0/0
Service-policy output: BANDWIDTH
Class-map: FROM-SW2 (match-all)
801601 packets, 928837110 bytes
30 second offered rate 10266000 bps, drop rate 3520000 bps
Match: access-group 40
Queueing
queue limit 64 packets
(queue depth/total drops/no-buffer drops) 64/211287/0
(pkts output/bytes output) 590316/608962024
bandwidth 5% (500 kbps)
Class-map: class-default (match-any)
2675 packets, 2579856 bytes
30 second offered rate 227000 bps, drop rate 0 bps
Match: any
queue limit 64 packets
(queue depth/total drops/no-buffer drops) 0/0/0
(pkts output/bytes output) 2742/2475487
Router2(config-pmap-c)#
The bandwidth command gives minimum reservations used during congestion, it
does not police traffic at the configured rate. During congestion both
traffic types will contend for the unreserved bandwidth.
The docs indicate that the 25% of interface bandwidth which is not available
by default, is reserved for best effort, control and layer 2 overheads. By
allocating 100% of the interface bandwidth to other classes you may run into
problems.
Paul.
On Fri, Dec 5, 2008 at 12:49 AM, Pavel Bykov <slidersv@gmail.com> wrote:
> 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
> ----------------
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