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3550 QoS Queue question

Hi,

What is the priority order?

I have enabled priority-queue out on int f0/1 so now Q4 should has the no1 priority. Wich is the second and the third?

My goal is to assign priority(queue) to DSCP values. For example: DSCP 34,36,38,48 values should have no1 priority, DSCP 30 value should have no2 priority and so on...

Is there any way to assign DSCP values directly to priority queue?

Thanks in advance!

Switch#show mls qos interface fa0/1 queueing

FastEthernet0/1

Egress expedite queue: ena

wrr bandwidth weights:

qid-weights

1 - 25

2 - 25

3 - 25

4 - 25    when expedite queue is disabled

Cos-queue map:

cos-qid

0 - 1

1 - 1

2 - 2

3 - 2

4 - 3

5 - 3

6 - 4

7 - 4

Everyone's tags (2)
4 REPLIES
Super Bronze

Re: 3550 QoS Queue question

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

If the 3550 uses CoS for egress queue selection, check your IOS version's QoS documentation for possible mapping of DSCP values to CoS values.

BTW, other than PQ, I don't recall 3550 egress queues being absolute priorities.  If not, you might get close to that kind of prioritization by setting bandwidth ratios, for the next 3 queues, as 81:9:1.

Re: 3550 QoS Queue question

Hello Istvan

Regarding to your questions:

What is the priority order?

The egress priority queue is queue 4. It doesn't existe the secondary priority queue or so. Other queues will be WRR weights for queue 1, 2 and 3.

Is there any way to assign DSCP values directly to priority queue?

It is not possible to assing DSCP values directly to the priority queue, however what you can do is to map dcsp values to COS value 5, and because Cos 5 is assigned to the priority queue the specified dscp values will obtain priority-out as well.

Hope this helps.

Regards.

Wilson.

Please rate useful posts.     


Community Member

Re: 3550 QoS Queue question

Hi,

Thank you for very helpful post however i have some more question.

How do i map DSCP to COS?

I issued the command in global config mode: mls qos map dscp-cos 20 22 to 5 however that did not make any changes in output of  sh mls qos maps dscp-cos

Where should i apply the QoS policy?

For example:

Packets marked with DSCP 20 and 22 are moving between PC1, PC2 and Internet.

Do i apply the QoS on SW1's FA0/2-3 and SW2's FA0/2?

Bandwith between SW1, SW2, R1 is 10 Mb/s

Bandwith between R1 and Internet is 8Mb/s

Bandwith between PC and SW is 100 Mb/s

Thanks in advance!

Super Bronze

Re: 3550 QoS Queue question

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Where should i apply the QoS policy?

That depends on your overall end-to-end QoS policy.  It also might depend whether you mean application of an actual policy configuration statement or other QoS configuration statements that support a logical QoS policy.

In general, edge port ingress QoS looks to analyze traffic.  It might validate ingress markings and perhaps mark or remark some traffic.  Once traffic is within your QoS domain, ingress traffic is usually just trusted.

Also in general, egress QoS deals with congestion management, both prioritization of transmission sequence and/or drop prioritization.

If you have a case where egress congestion isn't possible, for example egress to either of your PCs if they are the only PCs (as shown in your topology), you might not bother with egress QoS.  However, keeping track of where you need egress QoS can be more troublesome than just doing it everywhere.  Also doing it everywhere avoids issues with topology changes.  For example, if the link between the two switches was upgraded to 100 Mbps, you could have congestion to PC1 (receiving traffic from PC2 and the Internet, concurrently).

In other words . . .

Where should i apply the QoS policy?

Where ever it's needed to accomplish your QoS goals.

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