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Bronze

QOS ,CBWFQ question

Hi everybody.


Thanks everyone in this forum who taught me so much and helped me get where i am today. I really wanted to visit this forum as often as i used to but this new job is keeping me on my toes :)

 

I have a question for you guys on CBWFQ.( we are only focusing CBWFQ not LLQ) I am getting conflicting statements to how the CBWFQ  queues are serviced.

The book  i am reading says service algorithm not published by cisco, while the videos lecture I have been watching says it is round robin.

 

I appreciate your help

 

Have a great weekend!

 

 

1 ACCEPTED SOLUTION

Accepted Solutions
VIP Super Bronze

Hi Sarah,Long time no hear.

Hi Sarah,

Long time no hear. Hope, you are doing well and the new job is challenging enough for you :)

According to below link (document) each class will have a FIFO queue assigned to it.

From the document:

Packets satisfying the match criteria for a class constitute the traffic for that class. A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.

more info here:

http://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfconmg.html#wp1001203

HTH

Reza

2 REPLIES
VIP Super Bronze

Hi Sarah,Long time no hear.

Hi Sarah,

Long time no hear. Hope, you are doing well and the new job is challenging enough for you :)

According to below link (document) each class will have a FIFO queue assigned to it.

From the document:

Packets satisfying the match criteria for a class constitute the traffic for that class. A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.

more info here:

http://www.cisco.com/c/en/us/td/docs/ios/12_2/qos/configuration/guide/fqos_c/qcfconmg.html#wp1001203

HTH

Reza

Super Bronze

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Posting

Classes, excluding LLQ, are generally serviced relative to their weights relative to other active classes.

Given:

policy sample1

class A

bandwidth percent 25

class B

bandwidth percent 25

class C

bandwidth percent 50

If all classes were active, classes A and B should each get 25% and class C 50%, as expected.

However what if only classes A and B were active, or just classes B and C?

Classes A and B, if only ones active, would each get 50%.

Classes B and C, if only ones active, class B would get 1/3 and class C would get 2/3 (except on ASRs, where I understand "excess" bandwidth is proportioned differently).

What if given:

policy sample2

class A

bandwidth percent 10

class B

bandwidth percent 10

class C

bandwidth percent 20

My understanding is, policy sample2 would work like sample1 (except on ASRs).

What if given:

policy sample3

class A

bandwidth percent 25

class B

bandwidth percent 25

class class-default

fair-queue

If the above is pre-HQF, multiple flow queues will compete for bandwidth with the class queues for classes A and B.  Servicing is a bit complex, I recall someone had posted an article on the internet how queue weights are determined.  Also, BTW, part of the complexity, class-default FQ computes IP Precedence in weight determination.

policy sample4

class A

bandwidth percent 25

class B

bandwidth percent 25

class C

bandwidth percent 50

fair-queue

In HQF, any class can use FQ.  The aggregate of all the class flow queues is supposed to be "honor" the class bandwidth.  FQ in this case does not use IP Precedence.

 

Cisco doesn't always well/clearly document how CBWFQ services all its flows, but its operation does differ a bit on IOS version and/or platform.  (Personally, I find Cisco allowing different CBWFQ servicing, between pre-HQF and HQF CBWFQ, for the same exact syntax, disconcerning.)

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