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Community Member

Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Sorry guys,

Another QoS Question here.

I have a 6500 and QoS is globally disabled.

Does this mean that a 1p2q1t (just picked this as an example) would be running in FIFO mode? Ie, the the total buffer capacity available for use?

If I enable QoS globally on the switch, would the default config start to get used and for CoS 0, I now only get what is reserverd for CoS 0 (Asumme no classification, and all ports untrusted)

which is 30% (see default config below) - I assume this would start to carve the buffer up, and 70% would not be available.

! 1p2q1t

set qos txq-ratio 1p2q1t 50 30 20

set qos wrr 1p2q1t 5 255

set qos wred 1p2q1t tx queue 1 70:100

set qos wred 1p2q1t tx queue 2 70:100

set qos map 1p2q1t tx 1 1 cos 0

set qos map 1p2q1t tx 1 1 cos 1

set qos map 1p2q1t tx 1 1 cos 2

set qos map 1p2q1t tx 1 1 cos 3

set qos map 1p2q1t tx 2 1 cos 4

set qos map 1p2q1t tx 3 1 cos 5

set qos map 1p2q1t tx 2 1 cos 6

set qos map 1p2q1t tx 2 1 cos 7

Or, is it like CBWFQ where you configure a bandwdith and if the other buffers are not being used, it will use them?

Many thx

Ken

7 REPLIES
Blue

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Ken,

Weighted round-robin (WRR) does not explicitly reserve bandwidth. If there are no packets in a queue it is skipped. If all traffic is COS 0 it can use the full bandwidth of the port.

I have seen this proven out in the real world, with untagged COS 0 night time server backups running at over 90mb on QOS enabled 1p3q ports.

Please rate helpful ports.

Dave

Community Member

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

MAny thx Dave,

The thing I was wondering though, was not the wrr even though, man thx for letting me know that if the queue is empty, it is skipped, but it was a concern about the txq.

So, the default (what I beleive is the default) is as follows :- (lets pick a 2q2t)

Default Configuration

! 2q2t

set qos txq-ratio 2q2t 80 20

set qos wrr 2q2t 5 255

set qos drop-threshold 2q2t tx queue 1 80 100

set qos drop-threshold 2q2t tx queue 2 80 100

set qos map 2q2t tx 1 1 cos 0

set qos map 2q2t tx 1 1 cos 1

set qos map 2q2t tx 1 2 cos 2

set qos map 2q2t tx 1 2 cos 3

set qos map 2q2t tx 2 1 cos 4

set qos map 2q2t tx 2 1 cos 5

set qos map 2q2t tx 2 2 cos 6

set qos map 2q2t tx 2 2 cos 7

Cisco Recommendations in paper (http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration_09186a008049b062.pdf)

! 2q2t

set qos txq-ratio 2q2t 30 70

set qos wrr 2q2t 30 70

set qos drop-threshold 2q2t tx queue 1 40 100

set qos drop-threshold 2q2t tx queue 2 80 100

set qos map 2q2t tx 1 1 cos 1

set qos map 2q2t tx 1 2 cos 0

set qos map 2q2t tx 2 1 cos 2,3,4,6,7

set qos map 2q2t tx 2 2 cos 5

So in the first example, I am putting CoS 0,1,2,3 in Queue 1 and Queue 1 has a buffer allocation of the port of 80%.

The the second, CoS 0 and 1 go into Queue 1 with a buffer allocation of 30% of the port.

Now, if I run enable QoS on a large scale network, one would assume, this would be a major concern, or, if Queue 1 reaches it threshold of 30%, does it just tail drop or would it use buffers from queue 2?

Many thx,

Ken

Blue

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Ken,

If the port is not over subscribed all traffic will go. If you have 80mb of traffic egressing a 100mb port it won't matter that 60mb is Q1 COS 0 which is only allocated 30%. If you have 150mb/s of traffic your mapping is going to determine what COS traffic will be dropped.

Here is a good doc on 6500 WRR:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801091a5.shtml

Dave

Community Member

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Hey fella :)

Again Thx. So let me make sure I got this right and am really sorry if I am being a dummy.

If i config the port txq-ratio using the command

set qos txq-ratio 2q2t 30 70

If i send 60 Mb of CoS 0 (no other traffic), it will all get sent? Also if I sent 90Mb it would get sent. If I send 110Mb 10Mb has to get dropped and that would use the drop threshold or would wrr determined the dropping of packets.

Or, are we saying from the document you sent, all 6500 ports use weighted tail drop as standard before any other congestion mechanisms take place? (The Catalyst 6000 has implemented an advanced version of tail drop congestion avoidance that drops all packets with a certain CoS when a certain percentage of buffer filling is reached. )

I am really really sorry, i am not getting this?

Kindest ragrds, and thx for the continued input,

Ken

Community Member

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

I think I have this worded out now. Many thx for the help :))

One last point if I may,

Is RX queueing an important factor to consider, on switch-to-switch connections (not access) or just TX?

Many thx indeed,

Ken

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Hi,

Normally you don't need to consider RX queueing because this will only come into play if the fabric is congested - an extremely unlikely scenario.

HTH

Andrew.

Community Member

Re: Enabling QoS on a Switch - Do I lose Bandwidth Capacity?

Many thx indeed,

Ken

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