Solved: QoS - Nested Shaping ? 50% restriction ?

swapnendum · ‎05-29-2016

Hello Guys

We have a requirement listed below which we intend to solve using nested shaping-

1. Total WAN bandwidth = 8Mbps

2. Class A : Reserve 4Mbps ; ClassB : 1Mbps ; ClassC : 1Mbps; Rest class-default

3. Class A+B+C combined shouldn't exceed 7Mbps Outbound

To achieve this, we were planning to use Nested Shaping but it doesn't work. We can see that IOS will only allow us to use 50% of bandwidth on the middle policy (PM_CHILD).

Is there any such restriction ? Documented ?

IOS 15.2(4)M3

--------

E.g

-

Class-map CLASS-A-B-C

class CLASS-A

class CLASS-B

class CLASS-C

-

policy-map PM_GRANDCHILD -> these reservations are not allowed by IOS when aggregate is more than 4000 in total

class CLASS-A

bandwidth 1000

class CLASS-B

bandwidth 1000

class CLASS-C

bandwidth 4000

-

policy-map PM_CHILD

class CLASS-A-B-C

shape average 7000000

service-policy PM_GRANDCHILD

-

policy-map PM_PARENT

class class-default

shape average 8000000

service-policy PM_CHILD

-

int fa0/0

service-policy output PM_PARENT

-

Error:

Router(config-pmap-c)# class CLASS-C

Router(config-pmap-c)# bandwidth 4000

Insufficient bandwidth 4000 kbps for the bandwidth guarantee

Router(config-pmap-c)# bandwidth 2000

Insufficient bandwidth 2000 kbps for the bandwidth guarantee

Router(config-pmap-c)# bandwidth 1999 -> this value is allowed

Router(config-pmap-c)#

willwetherman · ‎05-29-2016

Hi,

This is a behaviour of HQF. Please see the following under section 'Nested Policy and Reference Bandwidth for Child-Policy'

http://www.cisco.com/c/en/us/td/docs/ios/qos/configuration/guide/15_1/qos_15_1_book/qos_frhqf_support.html#wp1089442

Hopefully I can explain this correctly.

In your example, if you do not define a bandwidth using the bandwidth command under the PM_CHILD classes, then HQF assigns an equal bandwidth to each of the classes using the PM_PARENT class-default shape rate as the reference bandwidth. There are two classes under policy-map PM_CHILD; CLASS-A-B-C and class-default (not visible). Each of these classes will be allocated an equal share of 8Mbps = 4Mbps.

This is why IOS will only allow you to use 50% of the parent shaped bandwidth for the PM_GRANDCHILD classes - 1% which is reserved for class-default. This will explain why you could add a bandwidth value of 1999 and not 2000

To resolve this you need to configure class CLASS-A-B-C with an explicit bandwidth guarantee, for example

policy-map PM_CHILD

class CLASS-A-B-C

bandwidth 7000

shape average 7000000

service-policy PM_GRANDCHILD

This should hopefully solve the issue

View solution in original post

willwetherman · ‎05-29-2016

Hi,

This is a behaviour of HQF. Please see the following under section 'Nested Policy and Reference Bandwidth for Child-Policy'

http://www.cisco.com/c/en/us/td/docs/ios/qos/configuration/guide/15_1/qos_15_1_book/qos_frhqf_support.html#wp1089442

Hopefully I can explain this correctly.

In your example, if you do not define a bandwidth using the bandwidth command under the PM_CHILD classes, then HQF assigns an equal bandwidth to each of the classes using the PM_PARENT class-default shape rate as the reference bandwidth. There are two classes under policy-map PM_CHILD; CLASS-A-B-C and class-default (not visible). Each of these classes will be allocated an equal share of 8Mbps = 4Mbps.

This is why IOS will only allow you to use 50% of the parent shaped bandwidth for the PM_GRANDCHILD classes - 1% which is reserved for class-default. This will explain why you could add a bandwidth value of 1999 and not 2000

To resolve this you need to configure class CLASS-A-B-C with an explicit bandwidth guarantee, for example

policy-map PM_CHILD

class CLASS-A-B-C

bandwidth 7000

shape average 7000000

service-policy PM_GRANDCHILD

This should hopefully solve the issue

swapnendum · ‎05-30-2016

Excellent answer, many thanks.

Had seen the URL earlier but didn't interpret it correctly. It makes perfect sense now.