I've been trying to figure out the impact to port throughout if the corporate QoS policy is applied to edge ports which only receive DSCP_0 tageed packets.
I've reviewed the current QoS config and have written a couple of paragraphs on ingress/egress policy and potential impact to traffic.
Could you be kind enough to sanitise my thoughts on this?
QoS policy (for 3750 / 2960)
mls qos map cos-dscp 0 8 16 26 34 46 48 56
mls qos srr-queue input bandwidth 70 30
mls qos srr-queue input threshold 1 80 90
mls qos srr-queue input priority-queue 2 bandwidth 30
mls qos srr-queue input cos-map queue 1 threshold 1 1 2
mls qos srr-queue input cos-map queue 1 threshold 2 0
mls qos srr-queue input cos-map queue 1 threshold 3 6 7
mls qos srr-queue input cos-map queue 2 threshold 2 3
mls qos srr-queue input cos-map queue 2 threshold 3 4 5
mls qos srr-queue input dscp-map queue 1 threshold 1 9 10 11 12 13 14 15 16
mls qos srr-queue input dscp-map queue 1 threshold 1 17 18 19 20 21 22 23
mls qos srr-queue input dscp-map queue 1 threshold 2 0 1 2 3 4 5 6 7
mls qos srr-queue input dscp-map queue 1 threshold 2 28 30
mls qos srr-queue input dscp-map queue 1 threshold 3 48 49 50 51 52 53 54 55
mls qos srr-queue input dscp-map queue 1 threshold 3 56 57 58 59 60 61 62 63
mls qos srr-queue input dscp-map queue 2 threshold 2 24 25 26 27 29 31 35 36
mls qos srr-queue input dscp-map queue 2 threshold 2 37 38 39
mls qos srr-queue input dscp-map queue 2 threshold 3 32 33 34 40
mls qos srr-queue input dscp-map queue 2 threshold 3 41 42 43 44 45 46 47
mls qos srr-queue output cos-map queue 1 threshold 3 4 5
mls qos srr-queue output cos-map queue 2 threshold 1 2
mls qos srr-queue output cos-map queue 2 threshold 2 3
mls qos srr-queue output cos-map queue 2 threshold 3 6 7
mls qos srr-queue output cos-map queue 3 threshold 3 0
mls qos srr-queue output cos-map queue 4 threshold 3 1
mls qos srr-queue output dscp-map queue 1 threshold 3 32 33 34 40 41 42 43 44
mls qos srr-queue output dscp-map queue 1 threshold 3 45 46 47
mls qos srr-queue output dscp-map queue 2 threshold 1 16 17 18 19 20 21 22 23
mls qos srr-queue output dscp-map queue 2 threshold 1 27 28 29 30 31 35 36 37
mls qos srr-queue output dscp-map queue 2 threshold 1 38 39
mls qos srr-queue output dscp-map queue 2 threshold 2 24 26
mls qos srr-queue output dscp-map queue 2 threshold 3 25 48 49 50 51 52 53 54
mls qos srr-queue output dscp-map queue 2 threshold 3 55 56 57 58 59 60 61 62
mls qos srr-queue output dscp-map queue 2 threshold 3 63
mls qos srr-queue output dscp-map queue 3 threshold 3 0 1 2 3 4 5 6 7
mls qos srr-queue output dscp-map queue 4 threshold 1 8 9 11 13 15
mls qos srr-queue output dscp-map queue 4 threshold 2 10 12 14
mls qos queue-set output 1 threshold 1 100 100 100 700
mls qos queue-set output 1 threshold 2 125 125 50 400
mls qos queue-set output 1 threshold 3 100 100 100 400
mls qos queue-set output 1 threshold 4 60 150 10 200
mls qos queue-set output 1 buffers 50 5 40 5
mls qos
additional interface commands#
srr-queue bandwidth share 10 10 75 5
srr-queue bandwidth shape 10 0 0 0
priority-queue out
mls qos trust dscp
Here are my thoughts
If a port has LAN QoS applied what will be the impact to traffic which enters port only tagged with default QoS marking – DSCP 0?
The pertinent config for CoS / DSCP 0 is listed below :-
mls qos map cos-dscp 0 8 16 26 34 46 48 56
mls qos srr-queue input bandwidth 70 30
mls qos srr-queue input threshold 1 80 90
mls qos srr-queue input priority-queue 2 bandwidth 30
mls qos srr-queue input dscp-map queue 1 threshold 2 0 1 2 3 4 5 6 7
mls qos srr-queue output dscp-map queue 3 threshold 3 0 1 2 3 4 5 6 7
mls qos queue-set output 1 threshold 3 100 100 100 400
mls qos queue-set output 1 buffers 50 5 40 5
srr-queue bandwidth shape 10 0 0 0
srr-queue bandwidth share 10 10 75 5
priority-queue out
For ingress queuing (where applicable) default QoS markings – DSCP / CoS 0, will be mapped to queue 1 threshold 2 which provides a threshold value of 90% and bandwidth guarantee of 70% and allocated 90% of the buffers. Bandwidth is guaranteed but not limited to and therefore each queue can scavenge any available bandwidth from its opposite ingress queue. Queue 1 is allocated 90% of port buffers. The threshold value indicates the level at which the switch will commence Weighted Tail Drop (WTD) once the ports buffers reaches 90% full of 90% buffer allocation.
Queue : 1 2
-----------------------------
buffers : 90 10
bandwidth : 70 30
priority : 0 30
threshold1: 80 100
threshold2: 90 100
Fig 1. Ingress Queue Settings
Egress queueing allows the use of 2 queuesets with 4 dedicated queue’s per each set. Default CoS / DSCP values (0) are mapped to queue set 1 queue 3 and use threshold 3 (100%). This allocates 40% of total buffer space with 100% of this buffer allocation reserved for use by Q3 a threshold of 100% of total buffer before WTD takes effect. However the queue can scavenge 400% of 40% from the common pool if available before WTD takes effect. This equates to the queue being able to scavenge 100% of the available buffer space.
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. 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 the srr-queue bandwidth shape or the srr-queue bandwidth share command is ignored (not used in the ratio calculation). Therefore for a port configured with priority-queue out there will be 90% of bandwidth available for egress (10+75+5) before WTD mechanism commences.
Queueset: 1
Queue : 1 2 3 4
----------------------------------------------
buffers : 50 5 40 5
threshold1: 100 125 100 60
threshold2: 100 125 100 150 reserved : 100 50 100 10
maximum : 700 400 400 200
Fig 2.Egress Queueset1 Settings
To summarise a port which will only see default QoS markings, DSCP/CoS 0, then both ingress and egress configurations would allow 90% of available bandwidth / buffer space to be used before WTD starts.
