Cisco QoS - Default Class Allocation

Question

I'm running new IOS 15.4.3(M2) on 2911 and 15.4.3(S3) on ISR4331s. I have the following QoS policy:

policy-map WAN-EGRESS-POLICY
 class CLASSIFY-AF41
    bandwidth remaining percent 25
 class CLASSIFY-AF31
    bandwidth remaining percent 30
 class CLASSIFY-AF21
    bandwidth remaining percent 15
 class class-default
     random-detect dscp-based

In prior IOS versions, Cisco would allocate 25% to the default class, which leaves you to allocate 75% of the remaining bandwidth (or changeable with the max reserved bandwidth command). Now, Cisco allows you to configure up to 100%, however, I'm not clear on what "percentage" that leaves the default class.

For example, I have 70% assigned to classes above. My questions is:

Assuming full 100% allocation by default, what is allocated to the default class if it's not specified?

Answer 1

You are not assigning 70% of the bandwidth because you are using the remaining keyword.

You are not reserving or dedicating the bandwidth you assign. If a class is not using all its bandwidth, other classes can use it, too. The default class gets to use any unreserved bandwidth that is not used. It could use up to 100% of the bandwidth, as can any of the classes, if it is not reserved. The bandwidth percent command doesn't exclusively reserve bandwidth.

Cisco has a paper describing how this works:

How Is Unused Bandwidth Allocated?

This section explains how the queueing system distributes any remaining bandwidth. Here is how the Class-Based Weighted Fair Queueing Feature Overview describes the allocation mechanism: "If excess bandwidth is available, the excess bandwidth is divided amongst the traffic classes in proportion to their configured bandwidths. If not all of the bandwidth is allocated, the remaining bandwidth is proportionally allocated among the classes, based on their configured bandwidth." Let's look at two examples.

In the first example, policy-map foo guarantees 30 percent of the bandwidth to class bar and 60 percent of the bandwidth to class baz.

policy-map foo 
  class bar 
    bandwidth percent 30 
 class baz 
  bandwidth percent 60

If you apply this policy to a 1 Mbps link, it means that 300 kbps is guaranteed to class bar, and 600 kbps is guaranteed to class baz. Importantly, 100 kbps is leftover for class-default. If class-default does not need it, the unused 100 kbps is available for use by class bar and class baz. If both classes need the bandwidth, they share it in proportion to the configured rates. In this configuration, the sharing ratio is 30:60 or 1:2.