From: David Bombal (davidbombal@davidbombal.com)
Date: Sat Jun 17 2006 - 19:34:18 ART
Hi Sami,
The bandwidth statement provides a minimum bandwidth under load. Otherwise,
if class-default is not explicitly assigned a minimum bandwidth guarantee
and you had a Scavenger class (for things like Napster) the Scavenger class
can still rob the class default of bandwidth. This is because of the way the
CBWFQ algorithm has been coded: If classes protected with a bandwidth
statement are offered more traffic than their minimum bandwidth guarantee,
the algorithm tries to protect such excess traffic at the direct expense of
robbing bandwidth from class-default (if class-default is configured with
fair-queue), unless class-default itself has a bandwidth statement
(providing itself with a minimum bandwidth guarantee).
However, assigning a bandwidth statement to class-default (on nondistributed
platforms) currently precludes the enabling of fair queuing (fair-queue) on
this class and forces FIFO queuing on class-default (this limitation is to
be removed with the release of Consistent QoS Behavior code). An additional
implication of using a bandwidth statement on class-default is that even
though 25 percent of the link is reserved explicitly for class-default, the
parser will not attach the policy to an interface unless the
max-reserved-bandwidth 100 command is entered on the interface before the
service-policy output statement. This is because the parser adds the sum of
the bandwidth statements (regardless of whether one of these is applied to
the class-default) and, if the total is in excess of 75 percent of the
links bandwidth, rejects the application of the policy to the interface.
I hope this helps.
David Bombal
CCIE, CCSI, CCIP, CCSP, CCDP
http://www.ConfigureTerminal.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Sami
Sent: 17 June 2006 17:42
To: Cisco certification
Subject: [Bulk] QoS
Group,
Below statemnet from CCO, could some one explaing when to use class-default
with fair-queue..according to CCO if you use with class-default with fair
queue or not => flow classified and given best-effort treatment .can some
one explin the firrence
If a default class is configured with the *bandwidth* policy-map class
configuration command, all unclassified traffic is put into a single FIFO
queue and given treatment according to the configured bandwidth. If a
default class is configured with the *fair-queue* command, all unclassified
traffic is flow classified and given best-effort treatment. If no default
class is configured, then by default the traffic that does not match any of
the configured classes is flow classified and given best-effort treatment.
Once a packet is classified, all of the standard mechanisms that can be used
to differentiate service among the classes apply.
Thanks
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