QoS Class-default - What happens when not configured?

[Logan Kampsnider]

I've been searching the Internet for an answer to this for quite some time and unable to find anything on it. What happens if you do not configure the 'class-default' command in a 'policy-map' that you apply to an interface? Will non-matched traffic get dropped? My understanding is the 'class-default class is designed to catch anything that isn't matched in previous 'class-maps' with the policy, and then give that traffic best effort delivery.

Thanks in advance for the help.

LK

[Krishnendu AR · Accepted solution]

Hi Logan,

When you define a 'policy-map', the 'class-default' is automatically created and any traffic that doesn't match any of the 'class-map' will automatically put into 'class-default' and the give best effort delivery. By default 'class-default' class is designed to give to flow based weighted fair queue for the traffic.

However, you can manually define the 'class-default' and then define a different action for it.

Krishna

[Joseph W. Doherty]

removed - redundant

[Joseph W. Doherty · Accepted solution]

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Posting

As Krishna correctly noted, even when you don't explicitly define 'class-default', there's still a class default.  However, regarding Krishna's comment about 'class-default', also using WFQ by default, isn't always true.  I believe with HQF CBWFQ, 'class-default' now defaults to FIFO, and I know fair-queue with HQF CBWFQ no longer uses WFQ, it's just FQ.

Regarding 'class-default' dropping traffic, also by default, pre-HQF's non default classes (combined) could not allocate more than 75% of the bandwidth (this could be changed, by configuration).  HQF no longer requires a configuration setting to allocate more than 75%.

[Ashutosh Upadhyay]

Guys, does that mean any packet that matches to class-default as catchall with get DSCP reset to DF? or will it continue to carry the TOS value until class-default>> set dscp default is configured?

[Giuseppe Larosa]

Hello Ashutosh,

if you want to have DSCP set to default 0 for class-default you need to configure it it is not automatic.

In a marking service policy is appropriate to mark to DSCP default all traffic belonging to 'class-default' otherwise you can have user traffic flows attempting to use EF marking for example.

Hope to help

Giuseppe

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Another wording of what Giuseppe is describing. . .

Any class can have any kind of traffic in it.  It's up to your policy to determine what traffic is directed to which class.  Often such classification is based on ToS marking, but it doesn't have to be, nor exclusively.  This also applies to 'class-default'.

Classes, by default, do not remark traffic, unless configured to do so.

BTW, DSCP markings, by default (except for original WFQ), don't guarantee any special treatment with CBWFQ.  Treatment is dependent on your QoS configuration.