Wednesday, October 14, 2015

Cisco Catalyst 3750 egress QoS - priority queue vs shaped


Source: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html#topic3

e-gress QoS default behavior


Default Scheduler Configuration:
The priority queue is disabled. Both the shaped and shared mode are configured for the SRR. Shaped mode weights override the shared mode value. Therefore, the net result is queue 1 is serviced in shaped mode and queues 2, 3, and 4 are serviced in shared mode. This means queue 1 is serviced with an absolute value that is (1/25) percent, or four percent, of the bandwidth. Queues 2, 3 and 4 are serviced at 25 percent of the bandwidth. If the bandwidth is available, then queues 2, 3 and 4 can be serviced at more than 25 percent of the bandwidth.
Distribution1#show mls qos int gigabitEthernet 1/0/20 queueing
GigabitEthernet1/0/20
Egress Priority Queue : disabled
Shaped queue weights (absolute) :  25 0 0 0
Shared queue weights  :  25 25 25 25
The port bandwidth limit : 100  (Operational Bandwidth:100.0)
The port is mapped to qset : 1

e-gress QoS behavior, when priority queue is disabled


Scheduler Configuration:
Rack1SW3(config-if)#srr-queue bandwidth share 1 75 25 5
Rack1SW3(config-if)#srr-queue bandwidth shape  3  0  0  0
Cisco Catalyst 3750 egress queue does not support Low Latency Queueing (LLQ). It supports priority queueing. When you configure priority-queue out, queue 1 is always serviced when it has a packet.
Rack1SW3(config-if)#srr-queue bandwidth share 1 75 25 5
Rack1SW3(config-if)#srr-queue bandwidth shape  3  0  0  0
Rack1SW3(config-if)#priority-queue out
When you configure this command, the SRR weight and queue size ratios are affected because there is one fewer queue participating in SRR. This means that weight1 in thesrr-queue bandwidth shape or the srr-queue bandwidth share command is ignored (not used in the ratio calculation).