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The carrier has given me a COS profile of 102: 80/10/10 80% RT for a new 100mb circuit. I've compiled the below as my QoS policy-map and bandwidth configs.

interface GigabitEthernet0/0
 speed 100
 max-reserved-bandwidth 100
 service-policy output ***

policy-map ***
class voice
  priority percent 80
  set ip dscp ef
class priority1
  bandwidth remaining percent 16
  set ip dscp af31
class priority2
  bandwidth remaining percent 2
  set ip dscp af21
class class-default
  fair-queue

I'm a bit confused on the nature of "bandwidth remaining", the way I understand it is as follows;

class voice
  priority percent 80 = **80mbs**
  set ip dscp ef
class priority1
  bandwidth remaining percent 16 = **3.2mbs, or 16% of the 20mb remainder**
  set ip dscp af31
class priority2
  bandwidth remaining percent 2 = **0.336mbs, or 2% of the 16.8mb remainder**
  set ip dscp af21
class class-default = **16.46mbs, aka "whatever is left"**
  fair-queue

Am I incorrect?

  • 1
    The COS profile nomenclature is foreign to me. I gather that the 80/10/10 are percentages applied to the non-realtime data to map them into COS 2 through 4. Meaning that 80% of the non-realtime traffic goes in COS2, 10% in COS3, and 10% in COS4. Correct? The ATT COS profiles are listed here: planner.bus.att.com/tab004.pdf – Stephen Craven Jan 20 '15 at 18:46
  • EF is COS 1, af31 is COS 2, af21 is COS 3. So its 80/10/10 COS 1-3. – HAL Jan 20 '15 at 20:02
  • Apologies, Steven. You were correct. – HAL Jan 21 '15 at 14:03
5

You are correct about the semantics of "bandwidth remaining", but the numbers are a bit different in IOS. IOS only allows QoS to reserve 75% of the bandwidth by default, which can be tuned with max-reserved-bandwidth.

What this means is that the bandwidth reserved for the voice class in your example is about 60%, and "bandwidth remaining" dices up the remainder.

As noted in the link, IOS > 15.0 and IOS-XE is a little different.

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  • If I'm understanding correctly; that means that I've configured it properly. I've overwritten the default reserve to be 100%. Which means I'm reserving 83.5% of the bandwidth of the interface. The ge0/0 interface is actually set to 100mbs. – HAL Jan 20 '15 at 20:05
  • Doh! Yes. I think I had the interface config "above the fold". – Mark Rogaski Jan 20 '15 at 20:50
  • Looks like my methodology was correct, but my Queue sizes were wrong. "Bandwidth remaining" works directly on the remaining bandwidth after Class voice. So you have a total 100% of 20mbs leftover to work with when you issue the "bandwidth remaining percent" commands. – HAL Jan 20 '15 at 21:29
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class voice
  priority percent 80
  set ip dscp ef
class priority1
  bandwidth remaining percent 80
  set ip dscp af31
class priority2
  bandwidth remaining percent 10
  set ip dscp af21
class class-default
  fair-queue

The above is actually the correct config. You are looking for 80mbs, 16mb, 2mb and 2mb Best Effort.

80% of the remaining 20mbs is 16mbs. This leaves 4mbs remainder available for priority 2 and Best effort.

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