ONT - bandwidth remaining percent

jezg76

A question from the ONT Official Exam Cert Guide...

Examine the output shown:
!
policy-map AutoQoS-Policy
 class AutoQoS-Voice
  priority percent 20
  set dscp ef
 class AutoQoS-Signaling
  bandwidth remaining percent 5
  set dscp cs3
 class AutoQoS-Transactional
  bandwidth remaining percent 40
  random-detect dscp-based
  set dscp af21
 class AutoQoS-Bulk
  bandwidth remaining percent 20
  random-detect dscp-based
  set dscp af11
 class AutoQoS-Scavenger
  bandwidth remaining percent 1
  set dscp cs1
 class class-default
  fair-queue
!

Based on this configuration, how much bandwidth would be allocated to FTP traffic if the policy was applied to interface Fa1/0/0?

A.) 5 Mb
B.) 8 Mb
C.) 20 Mb
D.) 1 Mb
E.) 40 Mb
F.) 16 Mb

Now they say A. with an explanation as follows:

The Bulk class includes FTP.  The Bulk class has been allocated 20% of the remaining bandwidth, after allocating 20% to the Priority queue, 5% to Signaling and 40% to Transactional.  20% percent of the remaining 26 Mbps is 5Mbs.

Now I was under the impression that bandwidth remaining percent was based off the available bandwidth, so I figure the following:

max-reserved-bandwidth is default 75% and the priority queue is at 20%, so our available bandwidth on a FE interface is 55000kbps (100000kbps - 45%)

Would that not mean that the AutoQoS-Bulk class would get 55000 * .2 = 11000kbps?

I'm just not seeing how they are using 20% of 26Mbps and getting 5Mbps.

I appreciate the help.

kryolla

after thinking about and reading up more on bandwidth remaining percent I have no idea how the book came up with that answer.

The interface bandwidth is 100,000 kbps

you subtract the LLQ and 25% for routing protocols etc from interface bandwidth

20% of 100,000 is 20,000 and 25% of 100,000 is 25,000
what is left is 55,000 to be divided among the different classes.

class 1 is 5% of 55,000 = 2,750
class 2 is 40% of 55,000 = 22,000
class 3 is 20% of 55,000 = 11,000
class 4 is 1% of 55,000 = 550

jezg76

Thanks for your response, kryolla. I see we are both tackling this exam. Good luck!

My question to your explanation would be, according to my readings, wouldn't that have to mean that the bandwidth remaining percent command would actually have to be bandwidth percent instead to figure it out as you did?

Maybe I am misinterpreting the difference between these two commands, which is entirely possible.

kryolla

This link explains it better and good luck to you


http://ardenpackeer.com/qos-voip/tutorial-what-is-the-difference-between-bandwidth-percent-and-bandwidth-remaining-percent/

from the link

The bandwidth remaining percent command takes a percentage of the available bandwidth not from the total reservable bandwidth (100% of the interface). The bandwidth percent command takes a percentage of the total reserveable bandwidth.

In summary, we have the total reservable bandwidth of 100%. The bandwidth percent, priority percent, and max-reservable-bandwidth command makes reservations from the total reservable bandwidth. The bandwidth remaining percent command makes reservations from the available bandwidth (whats left over of the total reservable bandwidth after the other reservations is treated as 100%).

According to this explanation 20% of 55,000 (available bandwidth) is 11,000 or 11 mbps which is what you got

Hopefully somebody can come up on how the book got that answer as I am curious as well