CBWFQ and the Class-Default Class Map

Hello all,

Per Cisco:

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.
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. 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.


So if I understand this correctly what they are saying is that the
class-default class map handles any other traffic that you do not
specifically put in your custom class maps (besides control traffic and
L2 overhead). My questions:

  1. What happens if I use the bandwidth command under class-default
    where the assigned bandwidth is less than the 25% of the interface
    bandwidth? Am I creating an issue for control traffic in this case?
    policy-map XYZ
      class class-default
      bandwidth percent 5
    int g0/0
      service-policy input XYZ
  2. What if my custom class maps do not account for FTP and suddenly
    users start transfering big files? In this case FTP would be handled by
    the class-default class map along with the important control traffic and
    L2 overhead. Will FTP cause issues to the network control traffic in
    this case?

Can someone please tell me if I misunderstood Cisco's statement
above? I personally see an issue with this architecture and honestly I
always thought the 25% of reserved bandwidth was for control traffic
only but under some unconfigurable class map and that the class-default
class map used bandwidth from the remaining 75% just like all other
custom class maps.

Thanks in advance


P.S. Trying here because the ATC forum section seems to be dead so I apologize for the duplicate post

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