From: Carl Timm (carl_timm@xxxxxxxxxxx)
Date: Wed Jul 03 2002 - 14:11:28 GMT-3
Hopefully this will help clear up some of the CAR versus CBWFQ questions.
CAR is a classification, marking, and policing mechanism. What this means is
that CAR can be used to classify traffic and mark the IP Precedence field,
DSCP field, ... The policing mechanism of CAR takes place when traffic
exceeds the specified traffic profile. If packets exceed this profile, CAR
can re-mark a packet and send it with less priority, drop the packet, or
continue on to the next CAR rate-limit line. The main difference between a
policing mechanism and a shapping mechanism is that the shaping mechansim
will use a queue to slow the rate at which traffic is being sent in order to
bring the traffic back into profile. Whereas, policing drops packets in
order to bring traffic back into profile.
CBWFQ is neither a policing or shaping mechanism. It is actually a queuing
mechanism. By default, CBWFQ will drop traffic in a tail drop fashion. This
means once a queue is full it will drop all traffic trying to enter the
queue. CBWFQ can be configured with WRED in order to provide congestion
avoidance. WRED will drop packets in a random fashion in order to avoid
congestion. This is what helps in the TCP global synchronization issue.
As I stated above, CBWFQ by itself only provides a queuing mechanism. CBWFQ
allows for certain queues to receive more service based upon the class of
the traffic in the queue. CBWFQ doesn't provide classification or marking
services by itself. In order to provide classification and marking,
Class-based Marking must be configured with CBWFQ. Looking at a CBWFQ config
may look like it is performing both, but it isn't. Class-based Marking is
configured in the Modular QoS CLI (MQC) during the configuration of the
Class-map. The class-map is then referenced in the policy-map. The
policy-map is where CBWFQ is configured.
So, in short, CAR and CBWFQ do not perform the same functions. I hope this
helps.
Carl Timm, CCIE# 7149
>From: James Self <j.self@wcom.com>
>Reply-To: James Self <j.self@wcom.com>
>To: Erhan Kurt <kurt@superonline.net>, ccielab@groupstudy.com
>Subject: RE: CAR vs CBWFQ
>Date: Tue, 02 Jul 2002 18:34:01 -0400
>
>Perfect explanation when and how to use traffic policy vs traffic shaping
>
>http://www.cisco.com/warp/public/105/policevsshape.html#selection
>
>and CBWFQ
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
>_c/fqcprt4/qcfcbshp.htm#xtocid3
>
>jself
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Erhan
>Kurt
>Sent: Tuesday, July 02, 2002 7:13 AM
>To: ccielab@groupstudy.com
>Subject: RE: CAR vs CBWFQ
>
>
>Look at the URLs for overview! Well, it's also open in exam :)
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/qos_
>c/qcprt2/qcdconmg.htm
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/qos_
>c/qcprt4/qcdpolsh.htm
>
>
>
>
>-----Original Message-----
>From: William Wong Kun Sing [mailto:wong_kunsing@solsis-eso.com.my]
>Sent: 02 Temmuz 2002 Sal} 13:27
>To: 'Colin Barber'; ccielab@groupstudy.com
>Subject: RE: CAR vs CBWFQ
>
>
>CAR is basically used for the traffic policing which to prevent the global
>sync (TCP slow start behaviour waste the bandwidth) so that the bandwidth
>will be fully utilised. CAR will not shape the traffic which is done by
>GTS
>or FRTS or what we call as traffic shaping.
>
>CBWFQ is a Congestion Management Tool which do the scheduling of packet
>forwarding. Yes, car is for ip only at the moment.
>
>
>-----Original Message-----
>From: Colin Barber [mailto:Colin.Barber@telewest.co.uk]
>Sent: Tuesday, July 02, 2002 3:49 PM
>To: ccielab@groupstudy.com
>Subject: RE: CAR vs CBWFQ
>
>
>I think CAR always 'shapes' your traffic as you defined. CBWFQ only
>'shapes'
>when there is congestion and therefore is more bandwidth efficient.
>
>Also does CAR only support IP?
>
>Colin.
>-----Original Message-----
>From: Ted Richmond [mailto:rich_ted@yahoo.com]
>Sent: 02 July 2002 07:52
>To: ccielab@groupstudy.com
>Subject: CAR vs CBWFQ
>
>
>I am totally confused now!!!
>Can someone tell me when to use CAR / CBWFQ. Looks
>like both can be used to allocate bandwidth to
>different connections.
>
>Thanks.
>
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