QoS DSCP processed in order or defined as to what is dropped

[romanroma]

I am new to QoS concepts, and just reading the following link:

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/qos/configuration/guide/nexus1000v_qos/qos_6dscp_val.pdf

However, what I do not understand - is the DSCP processed in Decimal value order like in the chart or is it based on the 'drop probability' column if frames are going to get dropped or out of order?

I am rolling out VOIP and the vendor is telling me to mark all VOIP traffic for their service as EF(46) and make my company's traffic as one of the following AF(xx) values.

At this point I will most likely just strip or remark the DSCP values going into my Data Center since it appears QoS was never configured; however, I will have to work with the Data Center team.

Also, do I need to apply QoS on routed links Packets or is just related to L2 Frames?

Thank yoiu

[Joseph W. Doherty]

". . . is the DSCP processed in Decimal value order like in the chart or is it based on the 'drop probability' column if frames are going to get dropped or out of order?"

Depends on what the QoS policy that's been applied.  Traditionally, network devices don't do any "QoS" unless configured to do so.  Some of Cisco's more recent devices, I believe, may do some basic QoS, by default.

As to what "should" happen, for various DSCP markings, there are recommendations, both from RFCs, and Cisco's (the latest of both, very similar), but how your DSCP markings are processed, is your choice.

Stripping, or remarking, your DSCP values, is also your choice.

"Also, do I need to apply QoS on routed links Packets or is just related to L2 Frames?"

Packets and frames (only with VLAN tags) both can support QoS markings.  Of the two, packet ToS can easily go end-to-end, and it supports more values than frames.  (Unless you really need L2 QoS, generally better to use L3 QoS.)

BTW, DSCP EF for VoIP, is the usual recommendation, although strictly speaking (pun not intended), only for the "bearer" packets.  VoIP control packets can be assigned a lower priority (I recall recommendation is something in the 4 markings.)

[romanroma]

@Joseph W. Doherty 

Hi Mr. Doherty - it has been a while since we spoke, and I hope you are doing well.

Thank you for your suggestions, and insight. So, if I understand you correctly, it really depends on the AF(xx) value and the drop probability in relationship to the specific type of traffic when dealing with my company's data traffic. You are correct, and that the vendor is requesting the voice frames to be DSCP EF; however, it does appear that video is DSCP AF41, control and signalling is DSCP AF31 and some misc traffic is DSCP AF21.

"packets and frames (only with VLAN tags)" so point-to-point routed links strips off the DSCP values and only SVI/VLAN support DSCP? - if I understand correctly?

I also sort have confused myself and noticed that (COS) Class of Service can be used as well, and it appears that a previous engineer used

 srr-queue bandwidth share 1 30 35 5
 priority-queue out
 mls qos trust cos

Oppose to the 'mls qos trust dscp"

What or how is COS used (in this context and in general)? It appears that a simple policy was used for bandwidth, and is not modifying the DSCP values, and I am not really sure this is configuration is working, since no policy groups/acl was created - just a big MLS QOS MAP but not referenced.

I am sort of picking up the pieces after 2 engineers.

I am trying to learn about QoS as to just reading from the vendors implementation paper.

Thank you

[Joseph W. Doherty]

For the AFxy's Y value, I recall the RFC suggested, for which markings get dropped sooner are: AFx3 >= AFx2 >= AFx1.  First thing to note, if I remember correctly, all three values might actually have the same drop probability.  Second thing to note, even if the drop probabilities are different, there's no standard for how different the probabilities might be.  Lastly, again, how to use the ToS is up to you.  How ToS might be used is suggestions/recommendations.  The strongest being using a ToS value of zero for best effort and using IPPrec 6 and 7 or CS6 and CS7 for really important packets dealing with network "stuff".  (E.g. Cisco marks BGP packets with IPPrec 6 / CS6.)

p2p links, at L3, as with all L3 packets can do what they want with the ToS.

L2 CoS, again, needs VLAN tagged frames.  SVI not needed.  Might be p2p or not.  (L2 CoS allows for QoS when working with only "pure" L2 switches.  Again, if device supports, generally better to use L3 ToS.)

Besides L2 CoS frames needing VLAN tags to carry the CoS tag, it's only 3 bits whereas the L3 ToS (currently) is 6 bits.  The latter allows for many more markings.