Output drops due to QoS on Cisco 2960/3560/3750 switches

Do you see incrementing output drops on some interfaces after configuring QoS on
your Cisco 2960, Cisco 3560 Series or Cisco 3750 switch?

Common Scenarios
• Some of the interfaces start experiencing output drops once QoS is configured on
the switch.
• Specific applications may experience degraded performance after configuring QoS
on the switch. Say IP phones start experiencing choppy calls.

Possible Reason
Once you enable QoS on the switch, some traffic may start getting lesser resources
than before (bandwidth or buffer) and hence may get dropped on the switch.

Troubleshooting steps
Step1> Identify the interfaces which carry outgoing data for the affected application
or are seeing incrementing output drops. Compare the interface output rate and the
interface speed and ensure that the drops are not due to overutilization of the link.
Switch#sh int gi1/0/1
<some output ommitted>
GigabitEthernet0/1 is up, line protocol is up (connected)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
!interface speed is 1000 mbps
input flow-control is off, output flow-control is unsupported
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1089 <<--!ensure these drops are incrementing
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 4000 bits/sec, 6 packets/sec
5 minute output rate 3009880 bits/sec, 963 packets/sec
!output rate is about 3 mbps while the interface speed is 1000 mbps.
1

Step2> Ensure that QoS is enabled on the switch. If it is not enabled, output drops are
not related to QoS and hence further steps mentioned here are irrelevant.
Switch#sh mls qos
QoS is enabled <<---QoS ip packet dscp rewrite is enabled

Step3> Identify the marking of the outgoing traffic that is getting dropped on the
interface.
Switch#sh mls qos int gi1/0/1 statistics
GigabitEthernet1/0/1 (All statistics are in packets)
dscp: incoming
------------------------------0- 4:
0
0
5- 9:
0
0
10 - 14 :
0
0
15 - 19 :
0
0
20 - 24 :
0
0
25 - 29 :
0
0
30 - 34 :
0
0
35 - 39 :
0
0
40 - 44 :
0
0
45 - 49 :
0
198910
50 - 54 :
0
0
55 - 59 :
0
0
60 - 64 :
0
0
dscp: outgoing
-------------------------------

0
0

0- 4:
5- 9:
10 - 14 :
15 - 19 :
20 - 24 :
25 - 29 :
30 - 34 :
35 - 39 :
40 - 44 :
45 - 49 :

0
0

2

0
0

0
0
0
0
0
0
0
0
0
0

0
0
0
0
0
0
0
248484

0
0
0
0
0
0
0
0
0

0
0
0
0
0
0
0
0
0

0

0
0
0
0
0
0
0
0

0
0
0

0

0
0
0

0
0

0
0
0
0
0
0
0
0
0

0
0
0
0
0
0
0
0
0

0

0
0
0
0
0
0
0
0

0

50 - 54 :
0
55 - 59 :
0
60 - 64 :
0
cos: incoming

0
0
0

0
0
0

0
0
0

0
0

0- 4:
2
0
5- 7:
0
0
cos: outgoing
-------------------------------

0
0

0

0

0- 4:
0
0
0
0
5- 7:
0
0
0
output queues enqueued:
queue: threshold1 threshold2 threshold3
----------------------------------------------queue 0:
248484
0
0
queue 1:
0
0
0
queue 2:
0
0
0
queue 3:
0
0
0

0

output queues dropped:
queue: threshold1 threshold2 threshold3
----------------------------------------------queue 0:
1089
0
0
queue 1:
0
0
0
queue 2:
0
0
0
queue 3:
0
0
0
Policer: Inprofile:

0 OutofProfile:

0

Note: Though you see queue 0-threshold 1 dropping packets, this actually
will be queue 1 in furthertroubleshooting as queue numbering is 1 to 4 in
further outputs.

Step4> Check the marking to output-q map on the switch to determine which queuethreshold pair
maps to the marking getting dropped.
In this scenario, queue1-threshold1 is mapped to dscp 46, which is getting dropped on
the interface.This means that dscp 46 traffic is being sent to queue1 and is getting
dropped because that queue hasinsufficient buffer or lesser CPU cycles.
3

Switch#sh mls qos maps dscp-output-q

Dscp-outputq-threshold map:
d1 :d2

0

1

2

3

4

5

6

7

8

9

-----------------------------------------------------------0:

02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01

1:

02-01 02-01 02-01 02-01 02-01 02-01 03-01 03-01 03-01 03-01

2:

03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01

3:

03-01 03-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01

4:

01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 04-01 04-01

5:

04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01

6:

04-01 04-01 04-01 04-01

Step5> There are two ways to tackle these drops. First method is by changing the
buffer and threshold values for the queue dropping packets. second method is to
configure the scheduler so that the queue dropping packets is serviced more often than
rest of the queues.
First let us see how we can change the buffer and threshold for the affected queues.
Let us check the buffer and threshold values associated with the queue identified in
step 4.
Note: Each queue set has the option to configure the buffer size and
threshold value for the four egress queues. Then, you can apply any one of
the queue sets to any of the ports. By default, all interfaces use queue-set 1
for output queues unless explicitly configured to use queue-set 2.

In this scenario queue 1 in queue-set 1 has 25% of the total buffer space and threshold
1 is set to 100%
Switch#sh mls qos queue-set
Queueset: 1
Queue :
1
2
3
4
---------------------------------------------buffers :
25
25
25
25
threshold1: 100 200 100 100
threshold2: 100 200 100 100
reserved :
50
50
50
50
4

maximum : 400 400 400 400
Queueset: 2
Queue :
1
2
3
4
---------------------------------------------buffers :
25
25
25
25
threshold1: 100 200 100 100
threshold2: 100 200 100 100
reserved :
50
50
50
50
maximum : 400 400 400 400

Step6> If you wish to change the buffer and threshold values just for the affected
interface, pleasechange queue-set 2 and configure the affected interface to use queueset 2.
Note: You can change queue-set 1 also but as all the interfaces by default use
queue-set 1, the changewill be reflected to all the interfaces.

In the next step I am changing queue-set 2 so that queue 1 gets 70% of total buffer.
Switch(config)#mls qos queue-set output 2 buffers 70 10 10 10

In next step i am changing queue-set 2, queue 1 thresholds. Both Threshold 1 and
Threshold 2 are mapped to 3100 so that they can pull buffer from the reserved pool if
required.
Switch(config)#mls qos queue-set output 2 threshold 1 3100 3100 100 3200

Step7> Check if the changes reflect under correct queue and queue-set.
Switch#sh mls qos queue-set
Queueset: 1
Queue :
1
2
3
4
---------------------------------------------buffers :
25
25
25
25
threshold1: 100 200 100 100
threshold2: 100 200 100 100
reserved :
50
50
50
50
maximum : 400 400 400 400
Queueset: 2
Queue :
1
2
3
4
---------------------------------------------buffers :
70
10
10
10
threshold1: 3100 100 100 100
5

threshold2: 3100 100 100 100
reserved : 100
50
50
50
maximum : 3200 400 400 400

Step8> Make the affected interface use queue-set 2 so that the changes take effect on
this interface.
Switch(config)#int gi1/0/1
Switch(config-if)#queue-set 2
Switch(config-if)#end

Confirm if the interface is mapped to queue-set 2
Switch#sh run int gi1/0/1
interface GigabitEthernet1/0/1
switchport mode access
mls qos trust dscp
queue-set 2
end

Check if the interface is still dropping packets.
Step9> We can also configure the scheduler to increase the rate at which queue 1 will
be serviced using the share and shape options. In this example Queue 1 alone will get
50% of the total CPU cycles and rest three queues will collectively get 50% of the
CPU cycles.
Switch(config-if)#srr-queue bandwidth share 1 75 25 5
Switch(config-if)#srr-queue bandwidth shape 2 0 0 0

Check if the interface is still dropping packets.
Step10> If the packets are still getting dropped, as a last resort we can enable priority
queue on this interface. This will ensure that all the traffic in the priority queue to gets
processed before any other queue.
Note:Priority queue is serviced until empty before the other queues are
serviced.By default on 2960/3560/3750 switches, queue 1 is the priority
queue.

Switch(config)#int gi1/0/1
Switch(config-if)#priority-queue out
Switch(config-if)#end

The marking getting dropped on the interface can be mapped so that it goes to queue
1, which is now the priority queue . In this way we ensure that traffic with this
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marking always gets processed before anything else.
Switch(config)#mls qos srr-queue output dscp-map queue 1 threshold 1 ?

<0-63> dscp values separated by spaces (up to 8 values total)
More related topics
Cisco Catalyst 3560 v2 Series Switches
Cisco 3750 to 3850 QoS Conversion Commands
How to do Speed Limits on Port for Cisco 2960 Switches?
The Difference between 3750 MLS QoS and 3850 MQC QoS
Cisco Catalyst 3750 v2 Series and Catalyst 3560 v2 Series Switches
What’s the difference between Cisco 2960 LAN Base and LAN Lite?
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