QoS - Default queue configuration

[ross.wiggins]

According to the 3750 QoS configuration guide, the output default queue sets (1 or 2) have 25% buffers in each of the 4 queues.

Does this mean that whilst using the default queue configuration, there is no benefit to classifying traffic into any specific queue as they all receive equal amount of buffer space anyway?

[acampbell]

Hi Ross,

Please see the egress section in this link

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html

Pay attention to the interface command "srr-queue bandwidth share ....."

Regards
Alex

[ross.wiggins]

Hi Alex,

Thanks for responding. 

I was wondering though if after simply enabling QoS is the default configuration (without the bandwidth commands etc) at that point enough to prioritise traffic in the event of congestion?

So would traffic marked as DSCP 46 be prioritised ahead of DSCP 0 or does it require (what I am referring to) as extra config such as the command you mentioned? 

[acampbell]

Hi Ross,

By simply enabling QOS,I take
it you mean just adding this to the existing config:-

!
mls qos
!

This turns on QOS but in a 3750 and many other switch models
this means that all traffic entering your switch interfaces is
actually UNTRUSTED and therefore bleached to DSCP 0.

When QOS is diabled all packets aentering the switch are left as they arrived
and with all pakets being treated equally with really just first in first out
switching.

May be before you turn on QOS you should look at the QOS section in
the correct config guide for your version of IOS.

http://www.cisco.com/c/en/us/support/switches/catalyst-3750-series-switches/products-installation-and-configuration-guides-list.html

With you looking at DSCP 46 (Usually Voice) you certainly want to be thinking
about setting up with a priority "Q" for VOIP. 

Regards
Alex

[ross.wiggins]

Apologies I should have gone a step further...so to re-cap, if QoS was enabled and trust boundaries set, at that point, will packets/frames marked with DSCP 46 (for example) be treated differently (say prioritised ahead of or given more bandwidth etc) to a packet/frame with DSCP 0?

If so, why?

If not, what is required to achieve that?

Would it require using additional configuration such as the one you mentioned in your first response? 

The main point in my question is trying to understand if any default configuration on the switch will treat traffic differently  (during congestion) once QoS is enabled and trust boundaries set

[ross.wiggins]

On this switch, which from looking through the running-config or startup-config, it does not have any specific commands referencing 'bandwidth' or 'priority' or 'srr-queue' and therefore (as far as my understanding is concerned) means it has just had 'mls qos' enabled and various interfaces configured with the appropriate trust boundary.

I am then believing that the queue config is using the default settings.

Attachment Queue-set_1' shows queue-set 1 config

Attachment 'Cos_outputq_threshold_map' shows the output of the command 'show mls qos maps cos-output-q'

Attachment 'Cos-dscp map' shows the output of the command 'show mls qos maps cos-dscp'

Looking at the mappings etc, it is my understanding that:

1. A packet marked with DSCP 48 will use Q4 & threshold 1

2. A packet marked with DSCP 24 will use Q3 & threshold 1

If that's correct (& again it is my understanding) that there is no difference in how the switch will treat that traffic and therefore, if congestion occurs, there is no benefit to marking traffic as DSCP 48 over DSCP 24.

Is my understanding correct?

Also, if we take the same comparison between a packet marked with DSCP 48 & DSCP 0, then the difference is Q2's threshold is higher (200 vs 100) so does that mean DSCP 0 traffic is actually treated better than DSCP 48?

Providing my understanding is correct; would that mean if I want to treat DSCP 48 higher than DSCP 0, I need to add some additional config, such as using the 'bandwidth' commands and/or 'priority' queueing commands?

[ross.wiggins]

Anyone able to respond to my last post? 

Thanks.

[Joseph W. Doherty]

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Posting

Does this mean that whilst using the default queue configuration, there is no benefit to classifying traffic into any specific queue as they all receive equal amount of buffer space anyway?

Yes and no.  It depends on what you're trying to achieve, your traffic and how you map it into the queues.

Also by default, I recall, each queue gets an equal share of the bandwidth.  (To one of your later questions, by guaranteeing a percentage of bandwidth, it may insure traffic in one queue has the bandwidth it needs.  For example, if supporting VoIP, although you want to prioritize VoIP bearer traffic, because of the need to minimize latency and jitter, for VoIP control traffic, instead, you mainly want to insure it's not dropped due to insufficient bandwidth.  Having such traffic in its own queue, might do that.)

Do know, 3750s, by default, reserve half the buffers to interfaces and half to a common pool.  This often results in higher drop rates.

[ross.wiggins]

Thanks for the info Joseph.

Are you able to answer the queries in my previous post?

I am looking to ensure I understand it all correctly...

[Joseph W. Doherty]

Sorry for the delay in responding.

Which specific post and/or questions?

[ross.wiggins]

No problem Joseph.

It was the questions in the post with the attachments/screen shots.