Traffic shaping - class of service aware

Answered Question
Jun 12th, 2008

Is the shaper class of service or DSCP aware? In the below example, policy-map 25Mbs-75_24 is applied outbound to a serial interface. It's a parent child policy where it first shapes to 25Mb and then calls the policy map.

How does a voice packet get handled within the shaper? If more than 25Mb of data is sent outbound, the shaper is active. If voice packets (ef) arrive, they are buffered along with the data. Does the shaper look at the ef marking of this voice packet and move it to the "beginning of the line" therefore the next time the tokens are refilled (Tc) the voice packet(s) would be the first to go out?

policy-map 75_24

class ef

priority percent 75

class class-default

set ip dscp default

fair-queue

policy-map 25Mbs-75_24

class class-default

shape average 25000000

service-policy 75_24

I have this problem too.
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Correct Answer by n.nandrekar about 8 years 5 months ago

Hi!

At parent level, you apply shaping to 25 mbps. This leads to the traffic en excess of 25 mbps to be queued. On this queues traffic, the queuing mechanisms and other things configured in the child policies are applied.

So your "priority percent 75" will actually be 75% of 25 mbps.

Thus, in short, the above policy will lead to total output traffic being 25 mbps, out of which 75% will be voice "ef" traffic sent with priority and the other 25% will be class default traffic (data and control plane).

So the shaper would just queue the packets and the mechanisms in the child policy will actually " move the voice packets at the beginning of the queue" till 75% bandwidth is consumed. Actually no packets are "moved to start of the queue but different queues are maintained and a schedular takes packets in a proportion and order specified in policy map such that it logically appears to be "moved to the start of the queue"

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a0080114326.shtml

From the above link "

The router prints this log message when an Ethernet subinterface is configured with a service policy that applies queueing without shaping:

router(config)# interface ethernet0/0.1

router(config-subif)# service-policy output test

CBWFQ : Not supported on subinterfaces

Note that the same rule applies to a Gigabit Ethernet subinterface.

c7400(config)# interface gig0/0.1

c7400(config-subif)# service-policy ou

c7400(config-subif)# service-policy output outFE

CBWFQ : Not supported on subinterfaces

In other words, you must configure a hierarchical policy with the shape command at the parent level. Use the bandwidth command for CBWFQ, or the priority command for Low Latency Queueing (LLQ) at lower levels. Class-based shaping limits the output rate and (we can assume) leads to a congested state on the logical subinterface. The subinterface than applies "backpressure," and Cisco IOS begins queueing the excess packets that are held by the shaper.

"

Also the " network processor processing stages" in the following link will give more details :

http://www.cisco.com/en/US/netsol/ns341/ns396/ns223/ns227/networking_solutions_white_paper09186a00801eb831.shtml

Hope This helps.

Regards,

Niranjan

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Correct Answer
n.nandrekar Thu, 06/12/2008 - 20:35

Hi!

At parent level, you apply shaping to 25 mbps. This leads to the traffic en excess of 25 mbps to be queued. On this queues traffic, the queuing mechanisms and other things configured in the child policies are applied.

So your "priority percent 75" will actually be 75% of 25 mbps.

Thus, in short, the above policy will lead to total output traffic being 25 mbps, out of which 75% will be voice "ef" traffic sent with priority and the other 25% will be class default traffic (data and control plane).

So the shaper would just queue the packets and the mechanisms in the child policy will actually " move the voice packets at the beginning of the queue" till 75% bandwidth is consumed. Actually no packets are "moved to start of the queue but different queues are maintained and a schedular takes packets in a proportion and order specified in policy map such that it logically appears to be "moved to the start of the queue"

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a0080114326.shtml

From the above link "

The router prints this log message when an Ethernet subinterface is configured with a service policy that applies queueing without shaping:

router(config)# interface ethernet0/0.1

router(config-subif)# service-policy output test

CBWFQ : Not supported on subinterfaces

Note that the same rule applies to a Gigabit Ethernet subinterface.

c7400(config)# interface gig0/0.1

c7400(config-subif)# service-policy ou

c7400(config-subif)# service-policy output outFE

CBWFQ : Not supported on subinterfaces

In other words, you must configure a hierarchical policy with the shape command at the parent level. Use the bandwidth command for CBWFQ, or the priority command for Low Latency Queueing (LLQ) at lower levels. Class-based shaping limits the output rate and (we can assume) leads to a congested state on the logical subinterface. The subinterface than applies "backpressure," and Cisco IOS begins queueing the excess packets that are held by the shaper.

"

Also the " network processor processing stages" in the following link will give more details :

http://www.cisco.com/en/US/netsol/ns341/ns396/ns223/ns227/networking_solutions_white_paper09186a00801eb831.shtml

Hope This helps.

Regards,

Niranjan

(please rate helpful posts)

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