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About QoS Questions

Dear Expert,


I have the following question. When I design a QoS for a devices, such as router A, i always hear such as 1P7Q2T.

My first question is: 

What is the meaning of "T"


My second question is:

How can I know how many "P" and how many "Q" and how many "T" that i can be used on device's egress interface? 


Thanks all



Everyone's tags (1)
Hall of Fame Expert

Re: About QoS Questions

Hello osw200051,

this question is platform specific.

>> i always hear such as 1P7Q2T.

This kind of capabilities encoding is used for linecards of modular multi layer switches like C6500, C6800.

These capabilities are hardware based and depends on the type of linecard the egress port is member of.



My first question is:

What is the meaning of "T"


T= Thresold it indicates how many thresholds are implemented in each queue 2T = two thresholds likely under the lower threshold no drop occurs and over the upper threshold all packets are dropped.

When the queue length is between the two thresholds a form of "simplified " WRED may occur with a fixed mark probability denominator. This can change on different linecards.


>> How can I know how many "P" and how many "Q" and how many "T" that i can be used on device's egress interface? 



Use show module to find out the exact model and part number of the module where the egress port is present then go to the modules datasheet for your platform.



some modern linecard allow to make a configuration between two different "HW QoS Schemas" see for example the following datasheet about the 8 40GE ports linecard for C6800



  1p7q4t (default)
  2p6q4t (configurable)


  1p7q4t (default)
  2p6q4t (configurable
As you can see the variation is just changing one standard queue as the second priority queue.


This kind of considerations do not apply to SW based routers like ISR that implement modular QoS.


Hope to help





Re: About QoS Questions

Thanks Giuseppe Larosa,

If your campus switch, such as C9300, it just mention "QoS scale entries = 5120". May I know what that mean?
Hall of Fame Expert

Re: About QoS Questions

Hello osw20051,


the number you have taken for C9300 is present in the C9300 datasheet in the performance  table 7


All the numbers present in this table refers to the whole platform and describes the TCAM capabilities:

total MAC addresses in CAM L2 MAC address table, max IPv4 routes, max IPv6 routes, max multicast routes, max ACL entries and MAX QoS scale.


It may be easier to explain looking at the max ACL entries first 5120 exactly like the QoS Scale.

This means that we can create ACLs on the device and we can apply these ACLs to a single port or to multiple ports. However, in hardware the total number of ACE lines (ACL lines ) is 5120.

To go on in explaining I think that an ACL with 10 lines that is applied to 15 ports will use 10*15 = 150 ACE entries in HW.

An ACL with 100 lines applied to two ports will use 200 entries of the total of 5120.


Similarly, if the C9300 supports modular QoS we can define policy-maps using different traffic classes and the same policy-map can be applied to multiple ports.

In hardware a policy-map using 8 classes applied to 10 interfaces should use at least 80 QoS entries.

( I am not sure how many HW entries are used by each class it can depend on the type of action performed).

We can say that the provided number for QoS Scale means that you can use modular QoS on all ports without great issues in a standard QoS setup.

Final note: these limits are not increased by stacking multiple C9300 switches of the same type. The stacking provides benefits on the forwarding numbers and total number of ports available by each member switch must be able to store the CEF table in its own TCAM.



as it happens on Catalyst 3850 and 3650 the support of modular QoS is to be thought for marking and policing.

C9300 supports also shaping see the last link below about manually configured QoS.

See the following note taken from the same datasheet : the egress queues are 8 per ports and use Shaped Round Robin.


Superior QoS: The Cisco Catalyst 9300 Series offers Gigabit Ethernet speeds with intelligent services that keep traffic flowing smoothly, even at 10 times the normal network speed. Industry-leading mechanisms for cross-stack marking, classification, and scheduling deliver superior performance for data, voice, and video traffic at wire speed. Superior QoS includes granular wireless bandwidth management and fair sharing, 802.1p Class of Service (CoS) and Differentiated Services Code Point (DSCP) field classification, Shaped Round Robin (SRR) scheduling, Committed Information Rate (CIR), and eight egress queues per port


The reasoning about policy-map usage of TCAM is still valid but the platform uses them in ingress to perform marking (in output for shaping if used).



There are two main options using auto-QoS or using QoS


For manual QoS see


Policy Map on Physical Port

You can configure a nonhierarchical policy map on a physical port that specifies which traffic class to act on. Actions can include setting a specific DSCP or IP precedence or CoS values in the traffic class, specifying the traffic bandwidth limitations for each matched traffic class (policer), and taking action when the traffic is out of profile (marking).


Hope to help






VIP Expert

Re: About QoS Questions

"When I design a QoS for a devices, such as router A, i always hear such as 1P7Q2T."

Giuseppe already mentioned this, but something like 1P7Q2T is generally only seen in reference to switches, routers (unless a "rebadged" switch, e.g. 7600) generally software QoS based routers will (now a days) mention CBWFQ.
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