3750 QoS

[Mark Bowyer]

What I want to do is, give video conferencing and telephony traffic priority. I have got my dscp marking going in and out in what I beleive are the priority queues, which I thought were serviced before any of the other queues. I topped the link out to see whether it worked and I could see that the dscp values were showing on the qos stats for the interfaces, but I could also see on the video conferencing unit that lots of video packets were being lost. So my question is, do I need to do some sort of policing or something in order to guarantee bandwidth for my realtime traffic? Here is my config:

mls qos

mls qos map cos-dscp 0 8 16 24 34 46 48 56

mls qos srr-queue input dscp-map queue 2 threshold 2  24 26 34 46

mls qos srr-queue output cos-map queue 1 threshold 3 5

mls qos srr-queue output cos-map queue 2 threshold 1 2 4

mls qos srr-queue output cos-map queue 2 threshold 2 3

mls qos srr-queue output cos-map queue 2 threshold 3 6 7

mls qos srr-queue output cos-map queue 3 threshold 3 0

mls qos srr-queue output cos-map queue 4 threshold 3 1

mls qos srr-queue output dscp-map queue 1 threshold 3 24 26 34 46

mls qos srr-queue output dscp-map queue 2 threshold 1 16

mls qos srr-queue output dscp-map queue 2 threshold 1 18 20 22

mls qos srr-queue output dscp-map queue 2 threshold 1 25

mls qos srr-queue output dscp-map queue 2 threshold 1 32

mls qos srr-queue output dscp-map queue 2 threshold 1 36 38

mls qos srr-queue output dscp-map queue 2 threshold 2 26

mls qos srr-queue output dscp-map queue 2 threshold 3 48 56

mls qos srr-queue output dscp-map queue 3 threshold 3 0

mls qos srr-queue output dscp-map queue 4 threshold 1 8

mls qos srr-queue output dscp-map queue 4 threshold 3 10 12 14

mls qos queue-set output 1 threshold 2 70 80 100 100

mls qos queue-set output 1 threshold 4 40 100 100 100

class-map match-any voice

match ip dscp 46

class-map match-any video

match ip dscp 34

class-map match-any control

match ip dscp 24

class-map match-any control-VoIP

match ip dscp 26

policy-map vvc-map

class voice

trust dscp

class video

trust dscp

class control

trust dscp

class control-VoIP

trust dscp

****Interface commands****

service-policy input vvc-map

priority-queue out

Does anyone have any ideas?

[acampbell]

Mark,

A very good place of reference is here:-
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND_40/QoSCampus_40.html#wp1098448

The suggestion for 3750 switches is that you do
police the inbound traffic on the interface via the inbound
service policy.
You should also set q limits for the outbound on the interface.

something like this:-

!
policy-map vvc-map
class voice
police 128000 8000 exceed-action drop
trust dscp
class video
police 1000000 8000 exceed-action drop
trust dscp
class control
police 32000 8000 exceed-action drop
trust dscp
class control-VoIP
police 32000 8000 exceed-action drop
trust dscp
class class-default
set dscp default
!
!
interface FastEthernetX/X/X
srr-queue bandwidth share 1 30 35 5
priority-queue out
service-policy input vvc-map
!

Hope this gives you some pointers

Regards,
Alex.
Please rate useful posts.

[Joseph W. Doherty]

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Posting

Your video units were detecting packet loss only when you "topped the link out" or also when you were not stressing the link?

How exactly did you top out your link?

Where does the 3750 register drops (if it does)?

If your video traffic is in PQ, you shouldn't be seeing any packet loss unless you're PQ offered bandwidth is more than the port's capacity, either permanently or temporarily.  If the latter, buffer tuning might mitigate temporary congestion drops.

BTW, I see you're also mapping control traffic to PQ.  Normally, you only map real-time traffic to PQ.

I also see you're trusting four DSCP markings.  What's your intent for other markings?

[Mark Bowyer]

I was seeing packet loss on the video conferencing unit its self, didnt check the 3750 for packet loss. I topped the link out by doing multiple file transfers. There was no packet loss when I wasnt topping the link out. At the moment we only want to priority queue the dscp markings I have specified. We dnt have plans to mark any other traffic.

So let me get this straight joseph, when there is congestion, if the traffic is going through he priority queue then none of the traffic should be dropped? Unless as you say, the bandwidth on the queue is more than the interface bandwidth. I shouldnt need to do any form of policing?


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[Joseph W. Doherty]

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

Maybe and yes.

PQ on 3750 is supposed to dequeue the PQ before any other queue.  Effectively, this is like giving that queue exclusive access to all the port's bandwidth.  If packets are being dropped, they shouldn't be dropped because of packets in other queues.  (Of course, I'm assuming your file transfer test packets weren't also in the PQ.  I'm also assuming you have correctly configured PQ end-to-end.)

I haven't carefully studied your bandwidth and buffer allocations, but it's possible waiting packets in other than PQ will consume common pool packets which could cause PQ to run short of common pool buffers for its buffering needs.  This might be addressed by increasing PQ's reserved buffers or setting other queue buffering parameters such that they cannot consume all the common pool buffers.

PS:

As to using policing, yes you can restrict other traffic bandwidth consumption doing that, but it restricts all the time, even when there's excess bandwidth.  If you organize your queuing resources such that when there's congestion, only the non-PQ traffic gets dropped, you're getting the best possible performance for that traffic (dynamically).  Whether those packets are dropped because they exceeded a policer's cap, or whether they drop because there's insufficient available bandwidth, causing queues to overflow, that traffic will be impacted similarly.  What you do want to do, is insure your PQ traffic isn't adversely impacted by other traffic.  As noted above, PQ gives you 1st dibs on bandwidth, but it alone doesn't guarantee buffer resources.

[Mark Bowyer]

Thanks for all the info Joseph, I take it I am correct in thinking that the input priority queue is queue 2 and the output is queue 1?

Also, the priority queue out command, I take it that is only used for the dscp markings I have identified in my class map?


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[Joseph W. Doherty]

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

The input PQ defaults to queue 2 but queue 1 can be selected.   PQ isn't on by default.

NB:  As the ingress queue is the stack ring, normally it would require lots of traffic to congest it.

Yes, egress uses queue 1 (or queue 0, depends on what stats you're looking at).

Both support default markings to queue, but you can remap.

[Mark Bowyer]

Ok and by default, which queues and thresholds would traffic use if they are not matched in my service policy? I guess I need to make sure that I am not limiting the bandwidth too much for all f my other traffic


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[Joseph W. Doherty]

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

Various QoS "show" commands would list your device's current settings.  Removing your explicit settings, would show the default settings.  (Don't have a device in front of me, running just defaults, that I could provide its stats.  Even if I did, there's some small chance if the device model is different or its running a different IOS version, defaults might differ.)