policy-map question

[billmatthews]

I have Cisco 2900 routers running IOS 15.1.  I have a pair of routers with a serial interface into an MPLS network (T1)...

LAN1 - R1 - MPLSWAN - R2 - LAN2

Here is the existing QoS config:

interface Serial0/0/0
 bandwidth 1536
 max-reserved-bandwidth 100
 service-policy input SERIAL-QOS-INBOUND
 service-policy output SERIAL-QOS-OUTBOUND
!
policy-map SERIAL-QOS-OUTBOUND
 class TAG-EF
  set dscp ef
  priority 480
 class TAG-AF41
  set dscp af41
 class TAG-AF42
  set dscp af42
  bandwidth remaining percent 10
 class TAG-AF31
  set dscp af31
  bandwidth remaining percent 10
 class TAG-AF21
  set dscp af21
  bandwidth remaining percent 25
 class TAG-AF11
  set dscp af11
  bandwidth remaining percent 10
 class class-default
  set dscp default
  bandwidth remaining percent 25
policy-map SERIAL-QOS-INBOUND
 class TAG-EF
  set dscp ef
 class TAG-AF41
  set dscp af41
 class TAG-AF42
  set dscp af42
 class TAG-AF31
  set dscp af31
 class Video
  set dscp af21
  police cir 768000
   conform-action transmit 
   exceed-action drop 
 class TAG-AF21
  set dscp af21
 class TAG-AF11
  set dscp af11
 class class-default
  set dscp default
!

We're having performance problems with video (af21).

Let's focus  on R2.  Am I reading this correctly that R2 will limit incoming video  (from R1) to 768k, and drop all traffic that exceeds that?  And then for  outbound video leaving R2, it will reserve it 25% of the bandwidth, but  beyond that it may experience drops? 

Thanks

[Giuseppe Larosa]

Hello Bill,

the policy map applied inbound implements a policer for class Video with CIR 768000 bps.

But there is also the following class-map that sets DSCP to AF21, is this wanted or it is just a typing error ?

The policy-map applied outbound should have a bandwitdth statement also for class TAG-AF41 as you are configuring a scheduler.

You have max-reserved-bandwidth 100%, the interface bandwidth set to 1536 kbps and a LLQ priority queue of 480 kbps.

when the link is full the class TAG-AF21 can get 25% of remaining bandwidth that is 25% of (1536-480 = 1056/4 = 264 kbps and this can be too little for video application

If the total usage of the link is less then 1536 the class TAG-AF21 can take some of the unused bandwidth and it can go over the 264 kbps. But if the link is fully used packets in excess of 264 kbps will be dropped and they will appear as output drops for the class in the output of

show policy-map interface ser0/0/0 output

Your understanding is correct

Hope to help

Giuseppe

[Joseph W. Doherty]

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Posting

Am I reading this correctly that R2 will limit incoming video  (from R1) to 768k, and drop all traffic that exceeds that?

Yes (if it matches the class).

And then for  outbound video leaving R2, it will reserve it 25% of the bandwidth, but  beyond that it may experience drops? 

If the class has insufficient bandwidth, it will queue, and if the queue fills, it drops.

Bandwidth reservations only really agree with what you see in the config when 100% has been allocated and all classes want at least their defined bandwidth.

The bandwidth reservations are really better understood as setting bandwidth proportions, or bandwidth ratios, between classes.  For bandwidth sharing, you would get similar results for:

class 1

bandwidth percent 25

class 2

bandwidth percent 25

class 3

bandwidth percent 50

vs.

class 1

bandwidth percent 5

class 2

bandwidth percent 5

class 3

bandwidth percent 10

As in both case the ratios are 1:1:2

If just classes 1 and 2 wanted as much bandwidth as the interface would supply, each would obtain 50%.

If just classes 1 and 3 wanted as much bandwidth as the interface would supply, class 1 would get 33.3% and class 2 would get 66.7%.

Only if all three classes wanted full interface bandwidth would you see 25%, 25% and 50%.  (Which conforms to the 25,25,50 configuration, but doesn't appear to conform to the 5,5,10 configuration.)

In your case, class TAG-AF41 doesn't define a bandwidth allocation, so I'm unsure what "percentage" it thinks it has.

Show policy interface will provide stats, which should show if your video is getting drops in ingress or egress.

For MPLS WANs, as they often are configured to support multipoint, a common performance problem is congestion coming out of the cloud (which is "invisible" to router stats).