I have following config for Qos.
Basically Access-list match the voip/signalling traffic & put that in High queue. I am trying to figure without hight utilization why I am seeing output drops in high queue.
7206#sh int s5/1/5:0
Serial5/1/5:0 is up, line protocol is up
Hardware is PA-MC-2T3+
Internet address is 172.17.133.105/30
MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec,
reliability 255/255, txload 24/255, rxload 23/255
Encapsulation HDLC, crc 16, loopback not set
Keepalive set (10 sec)
Last input 00:00:06, output 00:00:00, output hang never
Last clearing of "show interface" counters 01:58:03
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1800
Queueing strategy: priority-list 4
Output queue (queue priority: size/max/drops):
high: 0/20/1764, medium: 0/20/0, normal: 0/15/36, low: 0/15/0
5 minute input rate 143000 bits/sec, 82 packets/sec
5 minute output rate 145000 bits/sec, 83 packets/sec
734269 packets input, 148368640 bytes, 0 no buffer
Received 708 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
734855 packets output, 148838576 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions no alarm present
Timeslot(s) Used: 1-24, subrate: 1536Kb/s, transmit delay is 0 flags
7200#sh run int s2/1/5:0
Current configuration : 254 bytes
ip address x.x.x.x
no ip redirects
no ip unreachables
no ip proxy-arp
no snmp trap link-status
no cdp enable
Define access-list to match voice/signalling traffic
access-list 140 permit udp any any range 1024 1151
access-list 140 permit udp any any range 16384 32767
access-list 140 permit udp any any eq 2427
access-list 140 permit udp any any eq 2429
access-list 140 permit udp any any eq 5060
priority-list 4 protocol ip high list 140
priority-list 4 queue-limit 20 20 15 15
your queue limit is 20 packets. This can explain the behaviour. If there are 20 packets in the queueing system waiting to be serviced any newly arriving packet will be dropped.
Basically you can have drops in any queue in any queueing system if there is a permanent overload situation. There is no magic trick, if you f.e. permanently send 2 Mbps voip towards a T1. There will be drops.
QoS does not create resources, it just distributes them among your important traffic classes.
Besides this I also would suggest you to use class based queueing based on Modular QoS CLI (MQC). It is more flexible and precise than the old legacy methods like priority queueing, which you are using. On the other hand, if this is sufficient for you, why not.