Queuing concept

Hi Rene,

As i understand there are 5 main concepts in QoS
4. Shaping
I also understand that Cisco says for every platform (almost) there is default queuing mechanism exist , for example -
For 3850 it is 2P6Q3T queuing model , so if we deploy Cisco validated design for 2P6Q3T the whole QoS gets covered for this platform and we don’t need to do anything.
Now My Question is if we use such queuing model where does the classification and Marking go ? are they part of these queuing model ?

next small question is : I read somewhere that Cat 3850/3650 only support Egress queuing, what does that mean ? why not they support ingress queuing ?


Hello Abhishek

2P6Q3T is a queuing model. Thus, by default it encompasses only the Queuing concept of QoS. It does not include classification, marking shaping and policing. Marking and classification must be implemented in addition to this queuing model.

The 3850/3650 devices only support egress queuing. If however you want to implement some sort of QoS on ingress ports, it is possible to do so using other QoS mechanisms as described in this Cisco Documentation.

I hope this has been helpful!


If you want to check the queueing capabilities of your switch, you can find it on the interface level.

Here’s a 3560:

SW1#show version  | include IOS
Cisco IOS Software, C3560E Software (C3560E-UNIVERSALK9-M), Version 15.0(2)SE3, RELEASE SOFTWARE (fc1)

And its interface capabilities:

SW1#show interfaces capabilities 
  Model:                 WS-C3560X-48P
  Type:                  10/100/1000BaseTX
  Speed:                 10,100,1000,10000,auto
  Duplex:                half,full,auto
  Trunk encap. type:     802.1Q,ISL
  Trunk mode:            on,off,desirable,nonegotiate
  Channel:               yes
  Broadcast suppression: percentage(0-100)
  Flowcontrol:           rx-(off,on,desired),tx-(none)
  Fast Start:            yes
  QoS scheduling:        rx-(not configurable on per port basis),
                         tx-(4q3t) (3t: Two configurable values and one fixed.)
  CoS rewrite:           yes
  ToS rewrite:           yes
  UDLD:                  yes
  Inline power:          yes
  SPAN:                  source/destination
  PortSecure:            yes
  Dot1x:                 yes

In the QoS scehduling part, you can see the ingress queue (RX) is not configurable. The transmit (TX) queue is.

Thankyou Laz.
what will happen if I just implement this queuing model as-is and dont configure anything for classification and marking ?

Hello Abhishek

The 2P6Q3T uses the DSCP markings on traffic to implement its queuing policies. If nothing else is configured, then the default markings for the traffic are used. If these are tweaked, then of course the changes are taken into account.

I hope this has been helpful!


Thankyou very much. yes it clears now.