Bandwidth Remaining Percent command

Graham1

I was wondering about one of the questions on the CD-ROM in the ONT Official Exam Certification Guide dealing with the bandwidth remaining percent command. I think the answer on the CD-ROM migh be wrong:


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
!

Examine the output shown. Based on this configuration, how much bandwidth would be allocated to FTP traffic if the policy were applied to interface fa1/0/0?

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

The CD-ROM answer: E

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% of the remainng 26Mbps is 5Mbps.

20+5+40=65%, 65% of max-reserved (which is 75% of total bandwidt) reserved, leaving 35% of 75% of bandwidth remaining =

0.35*75=26.25

20% of this remaining 26.25Mbps =

26.25*0.20=5.25Mbps
**********************************

I think it should be:

priority percent 20

75 - 15 = 60
60 left now.

bandwidth remaining-percent 5 'allocate 5% of the remaining bandwidth.

60*0.05= 3Mbps

60- 3= 57Mbps left

bandwidth remaining-percent 40

57*0.40=22.8

bandwidth remaining-percent 20

57-22.8=34.2

34.2*0.20= 6.84


Available bandwidth = (interface bandwidth x maximum reserved bandwidth) – (sum of
all existing reservations)

% of remaining bandwidth would be a % of the available bandwidth.


I think the bandwidth that's been allocated to the other classes should be subtracted at each step since all the classes except AutoQoS-Voice are configured with bandwidth remaining percent, not bandwidth percent. Is this correct?

Forsaken_GA

I'm willing to bet the configuration on the CD-ROM has errors, as the explanation is acting like the Signaling and Transactional classes have been configured with the bandwidth percent command instead of the bandwidth remaining percent

unclerico

Using the formula:

Available bandwidth = (interface bandwidth x maximum reserved bandwidth) – (sum of
all existing reservations)


your interface is a 100mb/s interface using the default max-reserved-bandwidth. you have your priority queue configured to use 20% of the interface bandwidth. the calculation would be: (100mbps interface * 75% max-reserved-bandwidth) - (100mbps interface * 20% reserved for priority queue) = 55mb/s of available bandwidth. you use 55mb/s as the base for all of your remaining percent calculations
signaling = 5% of 55mb/s (2.8 mb/s)
transactional = 40% of 55mb/s (22mb/s)
bulk = 20% of 55mb/s (11mb/s)
scavenger = 1% of 55mb/s (550kb/s)
class-default = leftovers

Graham1

Thanks.

I have confirmed your explanation at the following link:

Low Latency Queueing with Priority Percentage Support [Cisco IOS Software Releases 12.2 T] - Cisco Systems

Forsaken_GA

unclerico wrote: »

Using the formula:

Available bandwidth = (interface bandwidth x maximum reserved bandwidth) – (sum of
all existing reservations)


your interface is a 100mb/s interface using the default max-reserved-bandwidth. you have your priority queue configured to use 20% of the interface bandwidth. the calculation would be: (100mbps interface * 75% max-reserved-bandwidth) - (100mbps interface * 20% reserved for priority queue) = 55mb/s of available bandwidth. you use 55mb/s as the base for all of your remaining percent calculations
signaling = 5% of 55mb/s (2.8 mb/s)
transactional = 40% of 55mb/s (22mb/s)
bulk = 20% of 55mb/s (11mb/s)
scavenger = 1% of 55mb/s (550kb/s)
class-default = leftovers

Right, this is why I think the configuration shown in the example is wrong, and only the Bulk class should have bandwidth remaining configured.

If you changed signaling and transactional to bandwidth percent instead, it breaks down like this -

20% + 5% + 40% = 65%. 65% of 75megs, is 48.75, leaving 26.25

If you then kick in the bandwidth remaining percent for the Bulk class, 20% of 26.25 is 5.25, which would indeed make E the correct answer.

Unfortunately, Cisco Press isn't known for their stellar editing

unclerico

Forsaken_GA wrote: »

Right, this is why I think the configuration shown in the example is wrong, and only the Bulk class should have bandwidth remaining configured.

If you changed signaling and transactional to bandwidth percent instead, it breaks down like this -

20% + 5% + 40% = 65%. 65% of 75megs, is 48.75, leaving 26.25

If you then kick in the bandwidth remaining percent for the Bulk class, 20% of 26.25 is 5.25, which would indeed make E the correct answer.

Unfortunately, Cisco Press isn't known for their stellar editing

Agreed!!!