Each port has a buffer. For egress there are 4 output queues.
With the following command you divide the buffer space among the 4 queues:
mls qos queue-set output <1/2> buffers 15 45 20 20 (sum = 100% of the buffer)
!<1/2> will be explained later
queue 1: 15%
queue 2: 45%
queue 3: 20%
queue 4: 20%
with the following command you map a cos value 6 and 7 to queue 2 thresshold 3:
mls qos srr-queue output cos-map queue 2 threshold 3 6 7
and with the following command you map a cos 3 to queue 2 and thresshold 2:
mls qos srr-queue output cos-map queue 2 threshold 2 3
So we mapped certain values to queue 2 but with different thressholds.
Thressholds and buffer usage is defined with the following command:
mls qos queue-set output <1/2> threshold queue thresshold1 thresshold2 reserved max
So for queue 2:
mls qos queue-set output <1/2> threshold 2 400 400 100 400
thresshold 1: 400%
thresshold 2: 400%
thresshold 3: 100% (implicit, not configurable)
reserved: 100%
max: 400%
thresshold means: if the buffer size is filled for x % start dropping traffic.
Thus if we have the following config:
mls qos srr-queue output cos-map queue 2 threshold 2 4
mls qos srr-queue output cos-map queue 2 threshold 3 6 7
mls qos queue-set output <1/2> threshold 2 40 70 90 400
When the buffer is filled with 70% of traffic it will start dropping traffic with cos 4 and not traffic with cos 6 or 7 because it's mapped to thresshold 3 (=100%)
the reserved and max value means:
[buffer size: mls qos queue-set output <1/2> buffers 15 45 20 20 ]
[mls qos queue-set output <1/2> threshold 2 40 70 90 400]
if you take for example queue 2. You configured 90% as reserved. So you reserve 90% of the buffer size of queue 2 (which was 45% of the total buffer space)
Thus 90% of the 45% of the buffer size is reserved.
The remaining 10% from queue 2 is seen as common buffer space. Can be used by queue 1, 3, 4
Max reserved means, how much it may expanded and use buffer space from the common pool buffer space as explained above.
You can create 2 sets. The set you define you indicate with the <1/2>. By this you can make two flavours.
By default all ports uses set 1.
mls qos queue-set output 1 buffers 15 45 20 20 ! queueset 1
mls qos queue-set output 2 buffers 5 5 5 80 ! queueset 2
as you see, there is a difference in how the buffers allocated to each of the four queues
int gig1/0/1
queueset 2
int gig1/0/2
queueset 1! default
so egress queue will use 15% for queue 1 on interface gig1/0/1 and 5% on gig1/0/2
Hope to help
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