From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Wed Jul 11 2007 - 16:23:13 ART
AFAIK, max-reserved-bandwidth is just an administrative limit.
It will not let you go over this "reservation" limit.
But regarding class-default, that's another story.
In the process of giving access to bandwidth, there are 3 "priorities":
-LLQ (AKA priority) goes first,
-Bandwidth allocated classes
-default class (if not explicitly allocated some BW).
So what the docs say is true, you can starve class default if you
oversubscribe the rest. And the numbers aare done in layer 3,
so the layer 2 headers are extra.
-Carlos
Jason Guy (jguy) @ 11/07/2007 16:03 -0300 dixit:
> I have been studying QoS some more, and I think the documentation
> contradicts itself in relation to the class class-default. In the
> Congestion management overview, it say this:
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
> fqos_c/fqcprt2/qcfconmg.htm#wp1001203
>
> CBWFQ Bandwidth Allocation
> The sum of all bandwidth allocation on an interface cannot exceed 75
> percent of the total available interface bandwidth. The remaining 25
> percent is used for other overhead, including Layer 2 overhead, routing
> traffic, and best-effort traffic. ***Bandwidth for the CBWFQ
> class-default class, for instance, is taken from the remaining 25
> percent.*** However, under aggressive circumstances in which you want to
> configure more than 75 percent of the interface bandwidth to classes,
> you can override the 75 percent maximum sum allocated to all classes or
> flows using the max-reserved-bandwidth command. If you want to override
> the default 75 percent, exercise caution and ensure that you allow
> enough remaining bandwidth to support best-effort and control traffic,
> and Layer 2 overhead.
>
> Then as I read through the WFQ configuration guide, it says this:
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
> fqos_c/fqcprt2/qcfwfq.htm#wp1017373
>
> Configuring the Class-Default Class Policy
> The class-default class is used to classify traffic that does not fall
> into one of the defined classes. Even though the class-default class is
> predefined when you create the policy map, you still have to configure
> it. ***If a default class is not configured, then traffic that does not
> match any of the configured classes is given best-effort treatment,
> which means that the network will deliver the traffic if it can, without
> any assurance of reliability, delay prevention, or throughput.***
>
>
> The interesting thing is that in one paragraph it basically says 25% of
> the interface is reserved (by default) for the class-default, L2, and
> control. Then the other says class-default is best effort. I would
> consider best-effort traffic to have to contend with the other Queue's
> for bandwidth. In this case, it seems the class-default is not in
> contention for the reservable 75% of the interface bandwidth (by
> default).
>
> Am I misinterpreting this? If you change the max-reserved-bandwidth to
> 95%, does this mean any traffic not classified has to contend for 5% of
> the control traffic? What if you put a bandwidth statement under the
> class-default class? Does that affect where it allocates it's bandwidth
> from? Seems like the class-default should be treated like any other
> class.
>
> Jason
>
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-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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