Qos using priority queue drops

Answered Question
Feb 12th, 2007

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+

Description: T1

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

non-inverted data

==============================================================================

7200#sh run int s2/1/5:0

Building configuration...

Current configuration : 254 bytes

!

interface Serial2/1/5:0

description T1

ip address x.x.x.x

no ip redirects

no ip unreachables

no ip proxy-arp

no snmp trap link-status

priority-group 4

no cdp enable

end

====================================================================================

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

I have this problem too.
0 votes
Correct Answer by mheusinger about 9 years 7 months ago

Hi,

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.

Regards, Martin

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spremkumar Mon, 02/12/2007 - 23:00

Hi

Is there any specific reason to have priority queuing in place in your router ?

Can you consider going on for LLQ instead of PQ ?

regds

Correct Answer
mheusinger Tue, 02/13/2007 - 01:26

Hi,

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.

Regards, Martin

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