Unanswered Question
Nov 23rd, 2007
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Hi all,

What's different between CBWFQ and DSCP. If use Sup32 in 7604, it seems CBWFQ not

support and DSCP is the only choice. Any sample configuration on it ?


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Overall Rating: 5 (1 ratings)
iskenderuz Fri, 11/23/2007 - 02:37
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CBWFQ and DSCP are different concepts. CBWFQ is a queing tool that reserves bandwidth for defined queues. DSCP is a 6-bit-field in the IP header carried end-to-end with default conditions/configurations. DSCP marks packets for the future actions on queues.

Also you can set DSCP values anytime with a route-map(CB marking) when CEF is enabled.

For instance, you can use DSCP values while configuring CBWFQ like below;

Router(config)# class-map class-xx

Router(config-cmap)# match ip dscp 30

Router(config-cmap)# exit

Router(config)# class-map class-yy

Router(config-cmap)# match ip dscp 10

Router(config-cmap)# exit

Router(config)# policy-map policy-zz

Router(config-pmap)# class class-xx

Router(config-pmap-c)# bandwidth percent 30%

Router(config-pmap-c)# exit

Router(config-pmap)# exit

Router(config)# class class-yy

Router(config-pmap-c)# bandwidth 20000

Router(config-pmap-c)# exit

Router(config)# interface xa/b

Router(config-if)# service-policy output policy-zz

Router(config-if)# exit

abdel_n Fri, 11/23/2007 - 03:38
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- DSCP (Differentiated Service Code Point) is a field in the IP packet header used for packet classification and marking, so each application (voice, video, mission-critical, transactional, best-effort, …) is given a DSCP value (marked) according to which it is classified.

- CBWFQ (Class-Based Weighted Fair Queuing) is an advanced software queuing mechanism based on WFQ with the support of user-defined traffic classes, CBWFQ classification is performed using the Cisco MQC (Modular QoS CLI) and applied only to output queue.

CBWFQ is used to classify data and the combination with a strict priority queue form LLQ (Low-Latency Queuing) which allow to optimally handle voice traffic and data traffic.

- Take a look at the enclosed pictures which I hope will give you an idea about those concepts.

Here is an example of MQC where different type of traffic (voice, http, streaming video and ftp) are classified by classes (CBWFQ for data + strict-priority for voice=LLQ), marked and given special treatment according to their DSCP marking.


class-map match-all sip-voice-policy

match protocol rtp

set ip dscp ef

class-map match-all http-policy

match protocol http

set ip dscp af11

class-map match-all rtspplayer-policy

match ip dscp default

set protocol rtsp

class-map match-all ftp-policy

match protocol ftp

set ip dscp af21

policy-map mypolicy

class sip-voice-policy

set ip dscp ef

priority 500

class ftp-policy

set ip dscp af21

bandwidth 150

random-detect dscp-based

class http-policy

set ip dscp af11

bandwidth 100

random-detect dscp-based

class rtspplayer-policy

set ip dscp default

bandwidth 50

queue-limit 50

interface Serial0/0

service-policy output mypolicy


- For more details about MQC: http://www.cisco.com/en/US/products/sw/iosswrel/ps5014/products_feature_guide_chapter09186a008008813a.html

- I know that both 7600 Optical Service Module (doesn't seem to be supported on sup32) and FlexWAN module (supported on sup32) with MSFC and PFC support CBWFQ.

- Qos on FlexWAN:


I hope this will help!


acbenny Sun, 11/25/2007 - 02:20
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Hi expert,

Can you show me the sample configuraton by using dscp

Condition (1)

If no congestion occur. There is no limitation on video and web traffic.

Condistion (2)

If congestion occur, video bandwidth will be reserved for 850Mbps and web bandwidth will be reserved for 150Mbps



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