QOS on 4948

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Jan 9th, 2009
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I would like an explanation on how the TX queue work on that peice of equipement.

If I do a sh qos int ten1/49, I get this:

QoS is enabled globally

Port QoS is enabled

Administrative Port Trust State: 'cos'

Operational Port Trust State: 'cos'

Trust device: none

Default DSCP: 0 Default CoS: 0

Appliance trust: none

Tx-Queue Bandwidth ShapeRate Priority QueueSize

(bps) (bps) (packets)

1 2500000000 disabled N/A 2080

2 2500000000 disabled N/A 2080

3 2500000000 disabled high 2080

4 2500000000 disabled N/A 2080

What`s the purpose of the bandwidth field?

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Jon Marshall Fri, 01/09/2009 - 09:14
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There are 4 queues on egress and each queue is allocted 1/4 of the bandwidth ie. 2.5Gbps in this case. You can change the allocation between the queues, or at least you can on some ports but i have never used a 4948-10G model.

You then map DSCP/Cos values to queues.

I suspect you may after something more specific. Attached is a link to Egress configuration on 4900 -


If i have not answered your question please come back.


dominic.caron Fri, 01/09/2009 - 12:11
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So, the high priority queue will be service first. The 3 other queue will share 1/3 each of the remaining transmit time. If I resume this,

1-If there is a packet in the high queue, put it on the tx-ring

2-take one packet from each other normal priority queue and place it on the tx-ring.

3-Start over

There is probably a check to see if the high priority queue is empty between each lower priority packet been put on the tx-ring.

Am I on the right track ?

dominic.caron Fri, 01/09/2009 - 12:15
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I've just read the link, got my answer, thanks.


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