Cisco SG-300 52 QoS default DSCP to queue mapping

[Nikolay Pertsev]

I am setting up QoS (advanced mode) on Cisco SG-300 52 switch.

I decided to go with DSCP to manage packets priority. While setting it up I found a configuration page (see the attached file for screenshot) where it is possible to map DSCP values to a particular queue. I did not quite understand the default settings of this mapping.

The queue assignment increases from 1 to 4 for DSCP values from 0 to 47. Well, the DSCP values 48 - 63 have the queue #3 assigned. In this way the packet with DSCP value 56 will have lower priority than packet with DSCP value 40. It does not make sense to me. Shouldn't priority (and thus queue) increase as the DSCP values increase?

Why the DSCP values 48 - 63 have queue #3 assigned as default settings?

[Tom Watts]

The queue is a default standardized value. As such you'll notice queue 4 is the most sensitive which is typically used for voice.  In order to use Advanced QoS you must use an access list to shape the ingress traffic to use the particular DSCP value. Here is a post that will help you if you're seriously trying to do this.

https://supportforums.cisco.com/discussion/11552296/cisco-sf300-qos-access-ports

0 = best effort

1 = class 1

2 = class 2

3 = class 3

4 = class 4

Per RFC 2597 there is an assured forwarding where the classes defined which would be called AF classes.

Drop	Class 1	Class 2	Class 3	Class 4
Low	001010 AF11 DSCP 10	010010 AF21 DSCP 18	011010 AF31 DSCP 26	100010 AF41 DSCP 34
Medium	001100 AF12 DSCP 12	010100 AF 22 DSCP 20	011100 AF32 DSCP 28	100100 AF42 DSCP 36
High	001110 AF13 DSCP 14	010110 AF23 DSCP 22	011110 AF33 DSCP 30	100110 AF43 DSCP 38

[Nikolay Pertsev]

HI, Tom and thank you for answer.

It all makes sense. The point of my question is why the DSCP values 48 - 63 have queue number 3 assigned while DSCP values 40 - 47 have queue number 4 which is of higher priority.

In this way the packet with DSCP value 56 (CS7) will have less priority than packet with DSCP value 40 (CS5).

Or am I missing something about DSCP and their relative value actually has nothing to do with their priority?

It would be logical to me if a packet with DSCP value 60 would go to queue #4 and a packet with DSCP value 44 will go to queue #3. Per default settings on my switch (see the screenshot) it is vise versa.

I understand that I can manipulate them as I want (that is why there is config page for this). But why the default settings propose such a config as shown on my screenshot.

[Tom Watts · Accepted solution]

I believe the answer to that direct query lies in RFC 2475

Here's the link to the RFC.

https://www.ietf.org/rfc/rfc2475.txt

 I think the info you're looking for is under the classifiers. I don't disagree that under any normal logic it should just respectively increment. For whatever reason, this was the decided standard/normal values.

I do know with the first 6 bits in binary has something to do with the decimal value (DSCP expressed in a number that's not binary).  I also think it has to do with how the values cycle in increments of 8.

Per RFC 791 here's the precedence values.

https://www.ietf.org/rfc/rfc791.txt

Example:

000 = Best Effort

001 = Priority

010 = Immediate

011 = Voice/Video

100 = Flash Over Ride