QoS/Queue Drops on 3750 switches

manten808 · ‎05-01-2013

Background: We currently have a site with a very simple topology that uses a 3750X switch stack for a collapsed core. Everyday, the users have a conference call and experience poor voice quality. Its not bad when users call from several conference phones, but when everyone calls in on individual phones, there is choppy and almost inaudible voice quality experienced. The voice traffic flow would be as follows: Phone <-> 3750 switch <-> Voice GW

We have packet captures showing that RTP packet loss is occuring from the phone to the voice gateway, but none from the voice gateway to the phones. We also have drops in the output queues that match drops on the asics. I can reset the counters and they will be clear until the call, and then they increment significantly during the call. The voice gateway and phones are non-Cisco. The switch stack has 6 switches. We are trusting the DSCP settings on the phones. All the queue drops from the phones are usually in queues 0-3, but all drops on the voice gateway is in queue 0. Below are the QoS settings; they are mostly default and we have not changed any queuing, thresholds, or buffers. Should we specify larger buffers and threshold for a designated queue and send EF traffic to that queue?

MySwitch#sh mls qos

QoS is enabled

QoS ip packet dscp rewrite is disabled

Typical Port

GigabitEthernet1/0/4

trust state: trust dscp

trust mode: trust dscp

trust enabled flag: ena

COS override: dis

default COS: 0

DSCP Mutation Map: Default DSCP Mutation Map

Trust device: none

qos mode: port-based

MySwitch#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 : 25 25 25 25

threshold1: 100 200 100 100

threshold2: 100 200 100 100

reserved : 50 50 50 50

maximum : 400 400 400 400

Interface directly connected to voice gateway

output queues enqueued:

queue: threshold1 threshold2 threshold3

-----------------------------------------------

queue 0: 30174799 0 0

queue 1: 163475 814 83213

queue 2: 42826 0 0

queue 3: 4265 0 2677

output queues dropped:

queue: threshold1 threshold2 threshold3

-----------------------------------------------

queue 0: 126997 0 0

queue 1: 0 0 0

queue 2: 0 0 0

queue 3: 0 0 0

Joseph W. Doherty · ‎05-01-2013

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Posting

Have you enabled PQ on the interfaces? If not, try that first.

If you have, or that alone doesn't fix your issue, then some tuning should be tried. If the drops are only seen on the egress port to the GW, assuming you're not using queueset 2, we can set that port to use that queueset and tune it independently of your other ports. For example, we might next try Q1, WTD1 at 200.