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QoS in MPLS Packet drops

Hello

I have experienced the following situation recently. It hasn't fixed yet. If you have any idea, please advise. Any comments are welcome!

The problem was that the voice packets have been dropped in class Voice despite the fact that the interface's utilization is quite low. Please see the policy-map interface output below.

Service-policy output: out_(3072k/3072k)_QoS (4754)

queue stats for all priority classes:

queue limit 192 (packets)

(queue depth/total drops/no-buffer drops) 0/297/0

(pkts queued/bytes queued) 12372/1041424

Class-map: Voice (match-all) (4755/7)

12098 packets, 1042365 bytes

5 minute offered rate 32000 bps, drop rate 3000 bps

Match: ip precedence 5 (4756)

Priority 50 (%) (1536 kbps) burst 1250000 (bytes)

I removed the service-module from the multilink interface and restored. I repeated the procedure for a few times. At some point, I noticed that queue-limit of LLQ changed from 192 to 384. I didn't change it manually. It was changed by itself. Then the drop stopped.

Service-policy output: out_(3072k/3072k)_QoS (6339)

queue stats for all priority classes:

queue limit 384 (packets)

(queue depth/total drops/no-buffer drops) 0/0/0

(pkts queued/bytes queued) 2628/221321

Class-map: Voice (match-all) (6340/7)

2653 packets, 228634 bytes

5 minute offered rate 14000 bps, drop rate 0 bps

Match: ip precedence 5 (6341)

Priority 50 (%) (1536 kbps) burst 1250000 (bytes)

* I omitted some of the output due to the text size in this forum.

The average packet sizes in queue-limit 192 and 384 are almost same (86 bytes/packet).

Does anyone know how does the IOS calculate the queue-limit in LLQ? I know that it changes dynamic in LLQ but would like to know the algorithm.

Did anyone have a similar experience?

Thank you,

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