Solved: CBWFQ and Ethernet WAN links

stretchlad · ‎07-15-2009

I can't seem to find any information on this but I always understood that fair-queue was for low bandwidth links i.e. sub 2Mb.

Is fair-queue viable as part of a CBWFQ policy on a 25Mb Ethernet WAN link? And should the conversations be altered as I undesrtand it it would default to 256 which doesn't sound a lot for a link that could support 100's of users.

Sample Policy:

Policy-map QoS

class VIDEO

priority 2000

Class class-default

fair-queue

random-detect dscp-based

random-detect ecn

Policy-map Shape

class class-default

shape average 23750000 237500 0

service-policy QoS

interface G0/1

service-policy output Shape

I would appreciate some advice on this.

Thanks

Joseph W. Doherty · ‎07-15-2009

Yes, original WFQ (interface fair-queue) is default on many serial interfaces, E1 and slower. However, CBWFQ policy map class fair-queue is different technology. It scales, and works, on higher bandwidth interfaces.

Regarding your concern about CBWFQ providing only 256 queues for hundreds of users, often many users don't have need for long term flows (not to be confused with sessions). Even on a congested interface, many users flows are likely short lived. So the issue than becomes, is some number such as 256 enough for concurrent active class flows? If it's not, results could be suboptimal, but then at that point you might need to reconsider either your AQM (active queue management) approach or whether there's need for more bandwidth.

View solution in original post

Giuseppe Larosa · ‎07-15-2009

Hello Kate,

what you get is WFQ on class-default with hierarchical QoS.

This is allowed and can be used

Hope to help

Giuseppe

Joseph W. Doherty · ‎07-15-2009

Yes, original WFQ (interface fair-queue) is default on many serial interfaces, E1 and slower. However, CBWFQ policy map class fair-queue is different technology. It scales, and works, on higher bandwidth interfaces.

Regarding your concern about CBWFQ providing only 256 queues for hundreds of users, often many users don't have need for long term flows (not to be confused with sessions). Even on a congested interface, many users flows are likely short lived. So the issue than becomes, is some number such as 256 enough for concurrent active class flows? If it's not, results could be suboptimal, but then at that point you might need to reconsider either your AQM (active queue management) approach or whether there's need for more bandwidth.

Istvan_Rabai · ‎07-15-2009

To complement to previous posts:

CBWFQ is not designed to have a queue for each separate flow or for each user.

It is designed to have a reasonable number of queues for traffic types that are present in the enterprise network, each traffic type requiring different treatment from the network.

And traffic types usually can be kept below 10, and in most cases it does not make much advantage to have several hundreds of traffic types and respective queues, as it complicates configurations, especially if we talk about end-to-end QoS.

Cheers:

Istvan

Joseph W. Doherty · ‎07-15-2009

"CBWFQ is not designed to have a queue for each separate flow or for each user. "

Yes and no, I think. CBWFQ is WFQ between classes, hence its name, but on most platforms it supported WFQ within class-default. On some hardware it supported FQ within most classes (e.g. 7500, 6500 FlexWAN). Now with HQF, many platforms support FQ within most classes. (FQ excluded for LLQ classes, I believe.)

Do agree you rarely need even 10 classes, although latest class models define 12, e.g. RFC 4594, Cisco (older) baseline 11.

However, within many classes, I've often seen benefit of FQ. Such configurations are not overly complex.

e.g.

Policy-map HQFexample

class real-time

priority percent 25

class critical

bandwidth percent 20

fair-queue

class bulk

bandwidth percent 4

fair-queue

class scavenger

bandwidth percent 1

class class-default

bandwidth percent 50

fair-queue

Istvan_Rabai · ‎07-15-2009

Hi Joseph,

Agree with you.

Thank you for your contribution.

Istvan

stretchlad · ‎07-16-2009

Thank you all for your very helpful responses. I feel alot more comfortable about this now :)