I am working on understanding the impact of using shaping and sharing with the SRR scheduler. Given these 2 commands on an interface:
switch(config-if)#srr-queue bandwidth share 40 20 30 20 switch(config-if)#srr-queue bandwidth shape 0 20 0 0
Gives me this in the configuration:
switch#show mls qos interface gigabitEthernet 1/0/5 queueing | include Sha Shaped queue weights (absolute) : 0 20 0 0 Shared queue weights : 40 20 30 20
I can understand the share command calculates the bandwidth like following:
With values that is:
So if i fill this with the values above, i get following bandwidth allocation for each queue:
Q1=36% Q2=18% Q3=27% Q4=18%
But now, the shaping command comes into play. Cisco documentation says following:
If you configure a weight of 0, the corresponding queue operates in shared mode. The weight specified with the srr-queue bandwidth shape command is ignored, and the weights specified with the srr-queue bandwidth share interface configuration command for a queue come into effect. When configuring queues in the same queue-set for both shaping and sharing, make sure that you configure the lowest number queue for shaping.
So the default configuration on an interface looks like this:
switch#show mls qos interface gigabitEthernet 1/0/6 queueing | include Sha Shaped queue weights (absolute) : 25 0 0 0 Shared queue weights : 25 25 25 25
So as far as I understand, the 25 shared weight for queue1 is ignored because this queue is being shaped. If queue2 is full, it can take unused weight from queue3 and queue4, giving it a max weight of 75. So does this mean, that if we enable QoS, leave all the settings at default and only have DSCP0 traffic (assigned to queue2 per default) we can utilize only 75% interface bandwidth?
Platform is a Cisco Catalyst 2960X switch.