QoS Bandwidth Calculation Question

jzangmeister · ‎11-08-2011

I'm trying to write a QoS policy for a branch router using CBWFQ to utilize the priority queue for voice. I already know how much bandwidth I need for each traffic class. So I am trying to calculate the percentage of BW required based on how much BW I need. I am having trouble with the percentage of traffic I am reserving matching up with the actual amount of BW I am trying to reserve.

From what I have read, when using the remaining bandwidth command, the percentage of bandwidth is calculated by the following logic: remaining BW = interface total BW - (priority queue + .25 * interface BW). The percentage allocated using the remaining bandwidth command is calculated using the remaining BW not the interface total BW.

If the above is true, then why aren't my numbers matching up with the BW in the show policy-map command? I have configured an Ethernet interface for 1536 BW.

1536 * .25 = 384 (25% non-reservable)

1536 * .21 = 322.56 (21% for priority queue)

384 + 322.56 = 706.56 (25% of int BW + 21% for priority queue)

1536 - 706.56 = 829.44 (remaining available BW)

I know I want to allocate 77kbps for voice signaling, which is 9% of the remaining BW. 77 / 829.44 = .0928. However, the show policy-map interface command shows a different amount of BW reserved for 9%. It shows 109 kbps. I believe it gets 109 kpbs by calculating the remaining BW from the interface BW - priority queue, not interface bandwidth -(non-reservable BW + priority queue BW).

Does anyone know why I am seeing this?

----------LAB Configuration----------

class-map match-any VOICE-TRAFFIC

match access-group name VOICE-TRAFFIC

class-map match-all DEFAULT

match access-group name DEFAULT

class-map match-any MISSION-CRITICAL-APPS

match access-group name MISSION-CRITICAL-APPS

class-map match-any NETWORK-MANAGEMENT

match access-group name NETWORK-MANAGEMENT

class-map match-any IP-ROUTING

match access-group name IP-ROUTING

class-map match-any CALL-SIGNALING

match access-group name CALL-SIGNALING

!

policy-map STORES-QOS-POLICY

description DSW Store Router QoS Policy

class CALL-SIGNALING

set dscp cs3

bandwidth remaining percent 9

class IP-ROUTING

set dscp cs6

bandwidth remaining percent 15

class MISSION-CRITICAL-APPS

set dscp af31

bandwidth remaining percent 28

class NETWORK-MANAGEMENT

set dscp cs2

bandwidth remaining percent 15

class DEFAULT

set dscp default

class VOICE-TRAFFIC

set dscp ef

priority percent 21

interface FastEthernet0/1

bandwidth 1536

no ip address

shutdown

duplex auto

speed auto

!

service-policy output STORES-QOS-POLICY

yourname#show policy-map int fa0/1
FastEthernet0/1

Service-policy output: STORES-QOS-POLICY

    queue stats for all priority classes:

      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0

    Class-map: CALL-SIGNALING (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name CALL-SIGNALING
        0 packets, 0 bytes
        5 minute rate 0 bps
      Queueing
      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0
      QoS Set
        dscp cs3
          Packets marked 0
      bandwidth remaining 9% (109 kbps)

    Class-map: IP-ROUTING (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name IP-ROUTING
        0 packets, 0 bytes
        5 minute rate 0 bps
      Queueing
      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0
      QoS Set
        dscp cs6
          Packets marked 0
      bandwidth remaining 15% (182 kbps)

    Class-map: MISSION-CRITICAL-APPS (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name MISSION-CRITICAL-APPS
        0 packets, 0 bytes
        5 minute rate 0 bps
      Queueing
      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0
      QoS Set
        dscp af31
          Packets marked 0
      bandwidth remaining 28% (339 kbps)

    Class-map: NETWORK-MANAGEMENT (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name NETWORK-MANAGEMENT
        0 packets, 0 bytes
        5 minute rate 0 bps
      Queueing
      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0
      QoS Set
        dscp cs2
          Packets marked 0
      bandwidth remaining 15% (182 kbps)

    Class-map: DEFAULT (match-all)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name DEFAULT
      QoS Set
        dscp default
          Packets marked 0

    Class-map: VOICE-TRAFFIC (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: access-group name VOICE-TRAFFIC
        0 packets, 0 bytes
        5 minute rate 0 bps
      QoS Set
        dscp ef
          Packets marked 0
      Priority: 21% (322 kbps), burst bytes 8050, b/w exceed drops: 0

    Class-map: class-default (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any

      queue limit 64 packets
      (queue depth/total drops/no-buffer drops) 0/0/0
      (pkts output/bytes output) 0/0

Joseph W. Doherty · ‎11-08-2011

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

"1536 * .25 = 384 (25% non-reservable)"

a) that can be changed on the interface - has it been?

b) 25% default was changed in later IOS versions, what IOS version are you running?

Your remaining percent number all appear to be BW - priority %.

jzangmeister · ‎11-08-2011

Answers to your questions:

a) I have not changed the max-reserved-bandwidth on the interface.

b) IOS 15.0(1)XA5

So, the new default is BW - priority %? What is the best practice for reserving BW then?

Joseph W. Doherty · ‎11-08-2011

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If I remember correctly, CBWFQ's default bandwidth reservation changed with 12.4(20)T, so your 15.0 version should be using HQF QoS.

Actually it might be BW - all defined bandwidths other than remaining, but I'm unsure.

You might just use defined bandwidths or straight percentages.

Marwan ALshawi · ‎11-08-2011

You can consider using hqos with a shaper in the parent qos policy

Plz refer to the below document for more details

https://supportforums.cisco.com/docs/DOC-8373

Hope this helps

If helpful rate

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