QoS PQ question

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May 17th, 2010
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Hi all,


I never before work with QoS and I need to try to solve one problem.

Problem is that I have high bandwidth Internet link on FE, but however sometimes 100% is used. I need to create QoS for certain server (IP address) so when link is used, and when server need to download something it need to have as much bandwidth as needed. I plan to use QP.

I try to configure like this

priority-list 1 protocol ip high list 122

priority-list 1 interface fastethernet 0/0 normal

priority-list 1 interface fastethernet 0/1 normal

priority-list 1 default normal

priority-list 1 queue-limit 20 40 60 80

Access-list 122 permit ip host aaa.bbb.ccc.ddd any

Access-list 122 permit ip any host aaa.bbb.ccc.ddd

And I apply this on both if FE0/0 and FE0/1.

Is this good configuration and it is possible to make prioritization for downloads at all?

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Calin Chiorean Mon, 05/17/2010 - 04:09
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Hi there,


As Cisco advices, I would also recommend not to use PQ, as if the traffic matched in the PQ will use the entier bandwidth, the other classes will starve, so the traffic will be dropped. If you really need to give more priority to a class I would suggest LLQ or CBWFQ.


Do you know how to configure MQS or you need an example?

bojan.vujic Mon, 05/17/2010 - 04:17
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Thank you on your answer.

Those Servers has been rate limited on other Cisco devices so can’t use all bandwidth.

Please can you give me configuration example of your recommendation?

And also is it my configuration correct?

Calin Chiorean Mon, 05/17/2010 - 05:27
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Your configuration is correct, but as I explain before you can have issue with this. Yes, your traffic (from the ACL 122) will always be prioritized in the high queue of PQ, but if you ALWAYS have this traffic, then the rest of the traffic which falls under medium, normal or low queue will never get delivered.


Next in regard to MQS, I have to understand what you want to do, because QoS can be applied input and output. I understand that you need download (?!) to have priority, but there is not too much that you can do on the input queue. The packets arrive on the input queue in the order (priority if you want) that are sent by the peer router (the one above your router in a vertical design) and then FIFO queueing is apply.


If both routers are under your administration (the one on which you applied PQ and the one above it) you can apply some CBWFQ on the top router, direction output on the interface connecting the lower router in which you can give priority to your "download".


Something like this:


1. - you make and ACL with the traffic you want to give priority

2. - you match the ACL in a class-map like this:


class-map match-all MYPRIO

match access-group ACL-NAME-OR-NUMBER-MADE-IN-STEP-1


3. -configure a policy-map in which you give priority to your traffic


policy-map POLICY

class MYPRIO

  bandwidth percent 50

class class-default


-if you have an 10Mbps interface, this will give 5Mbps to your prioritized traffic, the rest will be assigned to class-default in which enter all other traffic

-if you need to prioritize voice over IP (VoIP) than change the key "bandwidth" with "priority"; in this way you configure Low Latency Queueing (LLQ) which assure the delivery of the matched traffic before any other traffic


4. you apply the policy-map on the interface connecting the lower router direction output (this CANNOT be applied INPUT)


service-policy output POLICY


QoS is quite a complex topic and you have to know very well what you want to achieve and how you want to do that.


Good luck and let me know how it's working for you!


BTW, some helpful links:

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/fqos_c.html

http://www.cisco.com/en/US/docs/internetworking/technology/handbook/QoS.html

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