CBWFQ - How is a classes unused BW allocated during congestion

[Ahmed Dockrat]

Hi.

I've been reading the forum and the net trying to get a straight answer on this and can't seem to find anything concrete.

So basically If I have a CBWFQ setup for three classes, af3x (55%), af2x(30%) and class default(15%). applied outgoing on a WAN interface.

policy-map CBWFQ-POLICY
  class Af3x
  bandwidth percent 55
  random-detect
  class Af2x
  bandwidth percent 30

  random-detect
  class class-default
  bandwidth percent 15

  random-detect

Now I get  that in normal operation all classes will burst either way and during congestion, traffic in a specific class will be guaranteed it's bandwidth

.
But how is unused bandwidth from a class allocated during congestion. So for example if I have this link being congested primarily with Af2x and default traffic but have available capacity in Af3x, for this example say I have 30% AF3x still available.

Will the router split it  evenly15% Af2x and 15% default or will it split proportionatly based on the original BW allocations, ie 20% Af2x and 10% default.

Thanks

[Alessio Andreoli]

AF stands for assured forwarding and does not limit the amount of traffic that can go through. It actually is not that important anyway to define a  class as AF2x as much as defining a class as AF21 or AF22 etc...

Remember that AF21 has not real priority over AF31 or AF41. The "x" value is the one giving priority, the higher (e.g. AF33) the weaker (it will be drop first).

My suggestion anyway is to hierarchically shape the traffic in your classes.

HTH

Alessio

[Config T]

If your first class is not using all of its allotted bandwidth then the other two classes will use the available bandwidth based on the percentages you define in the other classes. That is, only if their bandwidth needs allow it. If your second class also doesn't need it then your default class will use it all if it needs it.

[Joseph W. Doherty · Accepted solution]

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Posting

What actually happens is the scheduler assigns weights to classes based on the bandwidth specification.  Class flows get their relative bandwidth based on the weight.  So, for example, if your class AF3x was using no bandwidth, and AF2x and class-default wanted all the bandwidth, they would split it 2:1 (30:15).

[Ahmed Dockrat]

Thanks for the help guys, this has cleared things up