QoS confusion

[techguy]

Hi,

Whats the purpose of following qos. What does it do? Thanks

class-map match-all Voice
 match dscp ef
!
policy-map QOS
 class Voice
  priority 4096
 class class-default
  fair-queue
policy-map Tag
 class Voice
 class class-default
  set dscp af43
policy-map shape
 class class-default
  shape average 20000000
   service-policy QOS
!
interface WAN
service-policy output shape
!
interface LAN
service-policy input Tag

[Martin Hruby]

Hello

In the configuration you've provided, there are 3 policy-maps.

Policy-map Tag is applied inbound on the LAN interface and it's purpose is to re-mark all received non-Voice packets (with DSCP other than EF) to DSCP = AF43.

Policy-map QoS provides priority treatment (PQ) to VoIP traffic (marked with DSCP = EF) up to 4Mb/s and fair-queueing to all other traffic. VoIP traffic exceeding 4Mb/s will be dropped so that other queues don't starve. Fair-queueing will ensure that no one greedy flow will consume all the bandwidth and starve other flows. The policy-map QoS is nested inside the policy-map shape, which shapes all outgoing traffic to an average rate of 20Mb/s. This is then applied on the WAN interface in the outbound direction. So interpret this as follows: 1) prioritize VoIP traffic up to 4Mb/s, 2) ensure all other flows are treated "fairly" and 3) shape all outgoing traffic (VoiP+all the rest) to 20Mb/s to meet the service-provider contracted traffic rate and prevent packet drops in the WAN due to packets exceeding contracted rate.

To verify the action of all QoS policy-maps, use show policy-map interface gi0/0

Best regards,
Martin

[techguy]

Excellent!!!

Can i say that this sate has 20 Mb out of 4Mb is dedicated to voice traffic. Is prioritization done here ? If so then how?

If this policy is in place do i need to do any Qos on LAN switch?

[Martin Hruby · Accepted solution]

Hello

Yes, you can say that the site has 20Mb/s of average traffic rate towards the WAN, and when VoIP traffic is present it gets priority treatment up to 4Mb/s. All other traffic then shares the remaining bandwidth fairly.

Whether or not you apply any QoS configuration on your LAN switch depends on your needs and your environment. One benefit would be to perform marking of packets on the LAN switch and then you could remove the policy-map Tag from the LAN interface of the router.

Best regards,
Martin

[Joseph W. Doherty]

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Posting

"Can i say that this sate has 20 Mb out of 4Mb is dedicated to voice traffic."

Dedicated, not exactly as other traffic could use bandwidth not being used by your Voice class.

"Is prioritization done here ? If so then how?"

Yes.  If average traffic rate exceeds 20 Mbps, the shaper will queue overrate packets.  The shaper will dequeue packets, to not exceed an average transmission rate of 20 Mbps.  LLQ queued packets will be dequeued first.  Also, if packets are queued, the 4 Mbps policer will be enforced on the Voice class packets.

"If this policy is in place do i need to do any Qos on LAN switch?"

Insufficient information to comment.  However, WANs links often have less bandwidth than LANs so QoS if often needed there "first".  LAN QoS is more likely to be needed when supporting VoIP or Video.

[techguy]

I understand that if the bandwidth (4mb) is not used by Voice then it will be free to be used by other traffic. If this is true, is it recommended to increase the percentage to 50% in low bandwidth sites (< 4MB). This way, we will give priority to more voice channels in these cases by squeezing internet browsing and email. Correct me please thanks

[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

Cisco recommends you don't exceed 1/3 the bandwidth for LLQ.  This to insure non-LLQ traffic has sufficient bandwidth.  However, allocations really depend on your traffic's service level requirements.  That said, you do need to be careful of need to allocate too much bandwidth to LLQ, not just for the reason for Cisco's recommendation, but also to avoid self queuing in the LLQ class.  I.e. you don't want LLQ to congest against itself.  At 50% you're probably okay.  It also depends on the VoIP codec being used.  Something like G.711 is constant while something like g.729 is variable.  The latter is more likely to self queue during bursts.

[techguy]

My understanding is that 50% will be reserveded by only voice traffic if we have LLQ or when we don't have voice traffic then all this bandwidth can also be used by non voice traffic.

[re775]

Hi Joseph W. Doherty,  Thanks for those great additional comments.

I also wanted to ask -- can you tell us if your comments apply to all platform supporting QoS, such as the Sup2T, ASR's, ISR's and so on?  Or your comment is only in reference to specific platform.

Thanks

[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

In theory, QoS is independent of device/platform.

In practice, QoS is very dependent on device/platform.  For example, many switches, including even the 6500, unless using a "WAN" line card like the FlexWAN, don't provide a shaper function.  This dependency can sometime require using a different device/platform.

[re775]

Thank you for the reply.

[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

Some clarifications:

"Policy-map Tag is applied inbound on the LAN interface and it's purpose is to re-mark all received non-Voice packets (with DSCP other than EF) to DSCP = AF43."

What the Voice is actually matching is not posted.  I.e. we don't know for sure that it's matching DSCP EF, although I would agree that's likely.

"VoIP traffic exceeding 4Mb/s will be dropped so that other queues don't starve."

Actually on many (most/all?) implementations, the implicit policer only activates when LLQ traffic is queued, i.e. you might have up to 20 Mbps of Voice class traffic.

[re775]

Great details Martin.  If I was to use "police cir percent xx" to get the 4mbps, instead of the " priority 4096", would your explanation change greatly?

policy-map QOS
 class Voice

     police cir percent XX

     class class-default
      fair-queue

Thanks in advance for your reply.