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New Member

QoS on c3750

I have 3 services accessible from servers attached to a 3750 switch.

Basically I would like to setup some prioritisation on the switch as 1 service is more important than the others.

I have been looking at QoS for this. I appreciate that it won't be an end-to-end QoS solution but want to be able to configure the switch so that it prioritises certain traffic flows. Can this be done? With QoS? Or is there a better approach.

I would like to use the 3750 as buying a seperate box is not really an option.

8 REPLIES
Cisco Employee

Re: QoS on c3750

Hi Leo,

You can ofcourse do this on 3750 using classification model and then applying proper policy to the classfied traffic.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12225see/scg/swqos.htm#wp1196344

HTH

Ankur

New Member

Re: QoS on c3750

Hi Ankur

Thanx for the response. I have looked at the link you provided. Is it possible to classify the traffic just based on IP address and port number using an extended access-list, as I guess the CoS and DSCP values in the packets will be the default.

Thanx

Leo

Cisco Employee

Re: QoS on c3750

Hi Leo,

Yes you can classify the traffic based on ip address and port numbers using extended access list.

you can perform the classification based on a configured IP standard or an extended ACL, which examines various fields in the IP header.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12225see/scg/swqos.htm#wp1044742

HTH

Ankur

* Please rate helpfull post

New Member

Re: QoS on c3750

Hi Ankur

Sorry, have little QoS experience myself.You can probably guess! What method would I then use to assign the QoS label as I obviously wont be looking at the DSCP or CoS fields.

Cheers

Leo

Cisco Employee

Re: QoS on c3750

Hey Leo,

After a traffic class has been defined with the ACL, you can attach a policy to it. A policy might contain multiple classes with actions specified for each one of them. A policy might include commands to classify the class as a particular aggregate (for example, assign a DSCP) or rate-limit the class. This policy is then attached to a particular port on which it becomes effective.

HTH

Ankur

New Member

Re: QoS on c3750

Errr.... QoS is well and good, if you really need it. Where do you want to apply the QoS? If you're thinking about prioritising within the switch, no chance! You can only prioritise traffic as it comes in or goes out of the switch.

Really, the question you should be asking yourself is: do I really need QoS? How much data will the servers really be pumping out? Will that amount of data saturate my uplink?

If the answers to the last two questions were no, then you don't need QoS. Bear in mind that Cisco QoS will only take affect when there is congestion on the link. If there is no congestion, then there's no QoS required. Just monitor your outbound ports over time, especially during peak times, and see if there're any drops. If the number of drops are increasing to an unacceptable level, then implement QoS. Otherwise, why makes things harder than necessary?

New Member

Re: QoS on c3750

Hi Michael

Thanx for your reply. Basically what I want to do is to prioritise traffic that goes through the switch. Some traffic that comes into a switchport is important whilst traffic that comes into another port may not be as important.

It is possible that the uplink could get saturated which would be unacceptable considering the other services. More likely is that it will slow down the other services - equally undesirable.

With the help from other posts I have a basic idea of classifying the traffic coming into a port using an ACL and then prioritising it by assigning it a DSCP value which will place it into the relevant queue.

I appreciate this isn't really the best way of using QoS but i've been asked to prioritise the different traffic flows into this switch.

Many thanks Leo

New Member

Re: QoS on c3750

Hi Leo.... One other thing you need to think about: when configuring QoS, make sure you configure both Layer-3 and Layer-2 QoS (i.e. DSCP and 802.1p CoS). This will mitigate any risk that a transit switch does not look beyond the layer-2 frame header when classifying a frame/packet.

QoS rule-of-thumb that we follow is:

1. Mark and Classify as close to the source as possible (edge/access switches, or Cisco IP Phones if you use these)

2. Only mark and classify specific traffic you're interested in (similar sort of concept as creating ACL)

3. Make sure CoS-to-DSCP and DSCP-to-CoS mappings are as per agreed industry standard (i.e. DSCP EF = 46, NOT 48. DSCP AF31 = 24, NOT 26. etc. etc.)

4. Mark a frame/packet with appropriate DSCP values AND the corresponding Layer-2 CoS for the class

5. Use as few classes as possible, for ease of management. No. of classes must not exceed 12 classes (Cisco's QoS Baseline), although most of the time 6-8 classes are plenty.

HTH.

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