ASR9K / PPPoe - Shaping

[vikas.hazrati1]

Hello all

We use ASR9k for PPPoE termination in our network. We have DSLAMs from various vendors but some (especially older models) have very limited buffer size per port. We wish to perform shaping / QoS on the ASR9K on PPPoE basis. So for e.g. subscribers that have a sync rate of 10Mbps we would apply something like:

policy-map shape-10M
 class class-default
  shape average 1000 kbps
  queue-limit 100 packets
 !

If the max buffer size in our DSLAMs is e.g. 100000 bytes I would like to configure the shaping accordingly (setting the burst size to 100000bytes) but no such parameter is present in the "shape average" command.

Is there any way to find (and if needed change) the burst-size for a shaping policy in ASR9k?

Thanks

V.

[Aleksandar Vidakovic]

hi Vikas,

if you want to configure a flat shaper on an interface on asr9k, we recommend to push it to parent level by configuring a dummy child policy, e.g.:

policy-map shape-10M
 class class-default
  service-policy Child
  shape average 10 mbps
!
policy-map Child
 class class-default
  bandwidth percent 100
!

The default queue size on asr9k is 100ms of service rate. In this case it would be 100ms worth of traffic at 10mpbs. In the above config, if you want to change the default queue limit, you should do it under the class-default config in the Child policy.

You can read more about queue limit setting and service rate in BRPSPG-2904 from Cisco Live Berlin 2016.

/Aleksandar

[vikas.hazrati1]

Hello Aleksandar

Thanks for your explanation, i will try to get access to the presentation you mention to see in more detail the qos in ASR9k.

One last question from the output of "sh qos-ea interface BE1000.100.pppoe43" I get the output:

-------------------------------------------------------
 Index 0 Level 0 Class name class-default service_id 0x0 Policy name shape-10M
 Node flags: DEFAULT 
 Stats flags: Queuing enabled 
 Node Config: 
 Shape: CIR/CBS/PIR/PBS: 0kbps/125000B/10000kbps/125000B 

The last line mentions CBS = 125000 bytes. For shaping 10Mbps => the max burst of the shaper (Bc) is 100msec? I was wondering if there is a way to decrease this even further (in same old DSLAMs the queue size < 100 packets for a particular port)

In older IOS/platforms we could define the Bc I was wondering if we could have something similar in ASR9K.

Thanks

V.

[xthuijs · Accepted solution]

hi vikas,

you can set the PBS (burst size) via 

RP/0/RSP0/CPU0:A9K-BNG(config-pmap-c)#  shape average 10 mbps ?

  <1-4294967295>  Excess burst size

this changes the PBS. CBS is somewhat erroneous in the shaper here, because the CIR is not set.

To explain the different variables and values:

- CIR is set by the bandwidth command (you can combine shape and badnwidth together, setting bandwidth/cir helps giving a minimum guarantee on that shaper in case there is oversubscription, eg sum of all PIR's is > interface bw).

- PIR is set by the shape command, not that you can still burst to line rate for PBS worth time

- PBS busrt size for the duration of peaking at linerate ABOVE the PIR.

The sanfran 2014 id 2904 has some details on that particular piece of how that graphs together.

The CBS would only come into play when there is oversubscription and we want to achieve our CIR as defined by bw, generally this is no factor.

cheers!

xander

[vikas.hazrati1]

Thnx for the explanation, I will give it a try in my setup