Default values should be ok for most environments. Also, Cisco writes that:
„Ingress congestion/buffer usually occurs during stack cable congestion.. In a non-stacked switch, you should not be experiencing packet drops due to congestion on the Ingress path”
Still, for any QoS fans, here’s a lowdown on ingress queueing:
On the ingress side, there are 2 queues, queue 2 is for voice traffic.
Queue 2 is the priority queue.
Queue 2 is allocated 10 percent of the interface bandwidth.
CoS 5 traffic is placed in queue 2.
This means that on a 1000mbit interface, 100mbit is allocated to your cos 5 traffic and it’s a priority queue. I really don’t see any potential problems here.
here’s how you can change the defaults for the ingress queue, though:
A) changing the buffers for queues:
mls qos srr-queue input buffers <queue 1> <queue 2>
mls qos srr-queue input buffers 70 30
B) configuring relative weights for traffic:
mls qos srr-queue input bandwidth <queue 1> <queue 2>
mls qos srr-queue input bandwidth 5 3
!!! this is a relative ratio only!!!
C) Configuring drop thresholds for each queue
mls qos srr-queue input threshold <queue no> <drop threshold no 1> <drop threshold no 2>
!Threshold 3 is always 100%!
This means that for e.g. queue no 1, drop threshold 1 will be 40%, drop threshold 2 will be 60%
D) Mappint cos values to thresholds
mls qos srr-queue input cos-map threshold 1 0 1 2 3
This means that traffic with COS 0,1,2,3 will be assigned to threshold 1, which means that at 40% traffic will start to get dropped
mls qos srr-queue input cos-map threshold 2 4 5
This means that traffic with COS 4 and 5 will be assigned to threshold 2, which means that at 60% traffic will start to get dropped
mls qos srr-queue input cos-map threshold 3 6 7
The following changes are not recommended unless tested thoroughly (why you would want to do it is beyond me)
E) If you want to totally change the mapping of queues to cos values:
mls qos srr-queue input cos-map queue queue-id threshold threshold-id cos1 cos2 cos3, eg:
mls qos srr-queue input cos-map queue 2 threshold 3 4 5 6 7
This will put traffic with cos values 4,5,6,7 to queue 2 (priority queue) and threshold 3 (drop only at 100% full)
F) You can also change which queue is the priority queue…
mls qos srr-queue input priority-queue queue-id bandwidth weight