From: Jason Guy \(jguy\) (jguy@cisco.com)
Date: Wed Jul 11 2007 - 16:03:29 ART
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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