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QoS Threshold Question - Catalyst 3750-X Series

Hi there,

I've inherited a small stack of Cisco Catalyst 3750's and have been tasked to understand and clearly document the QoS settings.

Personally I have a bit of Cisco experience, but no formal qualifications (currently reading the CCNET / CCNA books).

I've managed to figure out most of the stuff I need to know through reading various online documentation and forum posts, however some of the Thresholds in the Queues have got me stumped.

For example we have the following in one of the config files:

mls qos queue-set output 1 threshold 1 138 138 92 138

mls qos queue-set output 1 threshold 2 138 138 92 400

mls qos queue-set output 2 threshold 1 149 149 100 149

mls qos queue-set output 2 threshold 2 118 118 100 235

My understanding of this is:

Output Queue-set 1 Queue 1 threshold 1 drops at 138%
Output Queue-set 1 Queue 1 threshold 2 drops at 138%
Output Queue-set 1 Queue 1 reserves 92% of the buffer
Output Queue-set 1 Queue 1 has a maximum threshold of 138% of the total available buffers

Output Queue-set 1 Queue 2 threshold 1 drops at 138%
Output Queue-set 1 Queue 2 threshold 2 drops at 138%
Output Queue-set 1 Queue 2 reserves 92% of the buffers allocated to the queue
Output Queue-set 1 Queue 2 has a maximum threshold of 400% of the total available buffers

Output Queue-set 2 Queue 1 threshold 1 drops at 149%
Output Queue-set 2 Queue 1 threshold 2 drops at 149%
Output Queue-set 2 Queue 1 reserves 100% of the buffers allocated to the queue
Output Queue-set 2 Queue 1 has a maximum threshold of 149% of the total available buffers

Output Queue-set 2 Queue 2 threshold 1 drops at 118%
Output Queue-set 2 Queue 2 threshold 2 drops at 118%
Output Queue-set 2 Queue 2 reserves 100% of the buffers allocated to the queue
Output Queue-set 2 Queue 2 has a maximum threshold of 235% of the total available buffers

I understand that the Queue-set's can have a maximum of 400% of the total number of buffers as they can use memory that isn't being used (or reserved) for the other Queue-set's if required.

What I don't quite understand is how you can have a threshold above 100% as my understanding of thresholds is that they are a percentage of the available memory to the Queue-set.

Could it be that if the threshold is above 100% it tries to take free memory from the pool before dropping the frames?

Thanks,

Owen

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