[c-nsp] qos for broadband aggregation

[Tony]

Hi Mike,

In a nutshell:

* create qos policies on your aggregation router (7200/ASR). These should  
be a standard parent/child policy with the parent to shape the traffic and  
then whatever queueing you want in the child.
* add entries in radius to apply qos policy to subscriber  
virtual-interface when they connect

An example of the radius attribute returned:

Cisco-AVPair = "ip:sub-qos-policy-out=qos-dsl-8m"

which would reference the QoS policy defined on the router of "policy-map  
qos-dsl-8m".

The big problem you will have is knowing what speed to shape each  
individual DSL session to (ie. the downstream sync speed). There are some  
ways that this can be presented to you from the DSL carrier, but it's  
probably best to assume that you won't get this information (although  
lucky for you if it is presented to you). For QoS to be of ANY benefit you  
need to shape the traffic to LESS than the sync speed. This leaves you  
with a couple of options to achieve this outcome:

1. shape all to a uniform speed that you know will be achieved by all  
connections
2. shape to some speed you determine to be reasonable (but likely less)  
that the sync based on distance calculations
3. wait until service is up and check sync speed and apply the nearest  
speed shaper

We only do QoS for DSL services in limited numbers and so we use the third  
option. It would be manually intensive for any large number of services  
but is manageable for us. We also provide managed CPE (Cisco 800's) and so  
are able to determine the sync speed easily (and monitor it via SNMP  
should we choose to).

The problem is always shaping to something that is BELOW the sync speed so  
that your QoS functions, but not TOO much below so that you are robbing  
the users of performance. An extreme example would be that you could shape  
all sessions to 1mbps, but then if someone achieved a sync speed of  
20mbps, you'd be wasting the extra 19mbps that might be available to that  
user.

Without looking I think we have QoS policies for DSL of something like  
512k, 1500k, 2m, 4m, 6m, 8m, 10m, 16m, 20m. There might be a couple more  
between 10 & 20m, but you get the idea. Make sure you allow for your  
overhead as well, sync speed != QoS shaper (PPPoE/PPPoA, etc).

There is also the issue that sync speeds do change over time (usually  
downwards, rarely increasing), so we often choose to be conservative in  
case the speed of the service drops.

Good luck :)


regards,
Tony.


On Wed, 14 May 2014 07:30:14 +1000, Mike  
<mike-cisconsplist at tiedyenetworks.com> wrote:

> Hi,
>
> Does anyone have code snippets or pointers to implemeting QoS solutions  
> for broadband (eg: dsl) subscribers? The cisco docs are difficult to  
> read and real-world examples would be more helpful here.
>
> Thanks.
>
> Mike-