Applying QoS on 3560 Switch.

[aj_gonzalez1]

Hi everyone!

Im a bit confused about using MLS QOS in order to configure a determined bandwitdh percentage for specific traffic on these switches. I was wondering if anyone could point me on the right direction. Im a bit confused about the queues and their parameters.

I need to reserve 20% of available bandwidth for voice traffic. What would be the best way to archieve this on a 3560 switch?

Thanks :)

Edit: IOS Version is  12.2(44)

[Georg Pauwen]

Hello,

below is one way to do it. There are various configurations to accomplish the same result. If you have Cisco phones, you could also use auto qos. If you want to read up on that, check the link below:

mls qos
!
class-map match-any VOICE_CLASS
match ip dscp af31
match ip dscp cs3
match ip dscp ef
!
policy-map VOICE_POLICY
class VOICE_CLASS
priority percent 20
class class-default
fair-queue
!
interface FastEthernet0/0
service-policy output VOICE_POLICY

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swqos.html#95903

[aj_gonzalez1]

Thanks Georg! Ill certainly try doing that, because it seems waay simpler than using queue thresholds and DSCP maps.

[aj_gonzalez1]

I just tried doing this, but for some reason the command priority percent isnt available. Do I need to enable mls qos first before doing the rest?

[Georg Pauwen]

Hello,

what do you have at the prompt in the class map ? You could also use 'bandwidth percent 20', or 'priority' followed by the amount of kilobits per second that represent 20% of your bandwidth...

[Georg Pauwen]

On a side note, if you don't have Cisco phones, the DSCP values used might differ. Which ones do you have ?

[aj_gonzalez1]

Hi Georg,

Here are the options that I have available when trying to configure the policy-map:

Policy-map class configuration commands:
exit                     Exit from QoS class action configuration mode
no                      Negate or set default values of a command
police                 Police
service-policy    Configure QoS Service Policy
set                     Set QoS values
trust                   Set trust value for the class

Also, Im using both Cisco and other manufacturer phones, thats why I was gonna add an access list and mark traffic from those specific networks. 

[Georg Pauwen]

Hello,

my bad, you can only police, which is not what you want. Try auto qos as in the example below. Since you have Cisco and non-Cisco phones, the port configs vary slightly. In the example I have used interface ranges:

Non-Cisco Phones interfaces

3560# conf t
3560(config)# int range fa0/0-18
3560(config-if-range)# mls qos trust dscp
3560(config-if-range)# auto qos trust
3560(config-if-range)# priority-queue out
3560(config-if-range)# spanning-tree portfast
3560(config-if-range)# end

Cisco IP phones interfaces

3560# conf t
3560(config)# int range fa0/19-26
3560(config-if-range)# mls qos trust cos
3560(config-if-range)# mls qos trust device cisco-phone
3560(config-if-range)# auto qos voip cisco-phone
3560(config-if-range)# priority-queue out
3560(config-if-range)# spanning-tree portfast
3560(config-if-range)# end

[aj_gonzalez1]

Update:

So, I still havent been able to fulfill the original objective. Which is assigning a percentage of bandwidth to a queue.

Im still not sure if this is even possible, because all I keep seeing in the Cisco documentation are thresholds and buffer capacity configurations. But no bandwidth percentage anywhere.

Any help is appreciated.

[Georg Pauwen]

Hello,

unfortunately, on the 3560 (and the 2960) switches your only option with class based QoS is policing, which is not what you want.

The buffer/queue tuning is all you can do...

[aj_gonzalez1]

Hi Georg! Thanks again for taking your time to answer, really appreciate it.

Correct me if Im wrong, but policing traffic would just set a limit on how much traffic can be used by certain types of COS. That is indeed not what I want to do because I would need to guarantee said traffic percentage.

This policing might instead cause some dropped traffic if the buffers and dscp-maps are incorrect. Am I right?

[Georg Pauwen · Accepted solution]

You are absolutely right. Policing actually just cuts of excess traffic. If possible, shaping is much more preferable. But on the 3560 it is not available...

The auto qos settings I posted earlier ensure that your voice traffic gets prioritized...you can do it manually, but the auto generated values are usually good enough...