Re : QoS question ....

Unanswered Question
Feb 5th, 2009

[1] I have a 1MB link with 4 sub-interfaces. My question is that I plan to 'shape' one of the interfaces inbound to 1MB and leave the other sub-intefaces without shaping. If this is the case, would the other sub-interfaces get any bandwidth ?

[2] What if I shape average on all interfaces to 1MB inbound when the physical interface is only 1MB ? What would happen ?

Thank you,


- InternetB -

I have this problem too.
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Istvan_Rabai Thu, 02/05/2009 - 22:45

Hi InternetB,

Shaping does not work on interfaces inbound, only outbound.

The reason is that shaping uses buffers to delay traffic exceeding the defined rate.

If you want to use shaping inbound then you have to configure shaping outbound on the other router sending the traffic to this interface.

However, you can define policing on any interface inbound.



InternetB Fri, 02/06/2009 - 00:06

Hi Istvan,

Alright next question is that can I do the following outbound ?

[1] What if I shape average on ALL sub-interfaces (say I have 4 sub-interfaces) to 1MB outbound on all the sub-interfaces when the physical interface is only 1MB.

How does this work ?


- InternetB -

Istvan_Rabai Fri, 02/06/2009 - 00:57

Hi InternetB,

Suppose you have a Frame Relay connection with each subinterface serving a PVC.

You can configure shaping for each subinterface using Modular QoS CLI like this (a simple example):

policy-map SHAPE

class class-default

shape average 1000000 100000 0

interface s1/0.11

service-policy output SHAPE

The above policy will shape traffic to 1Mpbs, without the possibility to burst traffic above 1 Mbps.

Also, I did not configure adaptive shaping where the shaping policy slows down the shaping rate when a BECN is received.

So this is just a simple example.

You can reuse the same policy-map for all subinterfaces.



InternetB Fri, 02/06/2009 - 01:12

Hi Istvan,

Thanks again. My question was actually if the physical interface say is 1 MB and I have four-subinterfaces that I shape to 1MB. This would mean that I would effectively have 4MB of bandwidth which is not possible since the physical bandwidth of the interface is 1MB. How does the router detects this ?

I have tried this in dynamips and the router seems to take 1MB for traffic shaping when I apply it to all the 4 sub-interfaces ....


- InternetB -

Joseph W. Doherty Fri, 02/06/2009 - 04:44

I don't believe Cisco QoS is smart enough to determine multiple sub-interface shapers sum exceed available bandwidth.

If Cisco QoS doesn't detect shaping more than link capacity, it means your shapers might be ineffective.


If the physical interface is truly 1 Mbps, why shape at 1 Mbps? If the physical interface was more than 1 Mbps, but latter downstream you had only 1 Mbps, shaping to 1 Mbps makes senses. Again, though, if you have multiple shapers, you could exceed the downstream capacity if the shapers' sums exceed the available bandwidth.

You also might try a single shaper at the main interface; unsure if this can be done if there's subinterfaces. If it can, it would insure the aggregate bandwidth not exceeded. It wouldn't, though, allow you to set bandwidth proportions between the subinterfaces. That could be done by shaping each subinterface, such that the multiple subintface shapers don't exceed the available bandwidth.

Istvan_Rabai Fri, 02/06/2009 - 08:15

Hi Joseph,

Thanks for your additions.

One cannot use a shaper on a physical interface and on its subinterfaces simultaneously.

You're right, as the physical interface has 1 Mbps, one can either shape traffic on the physical interface or on the subinterfaces. But not both.

However (just and addition), you can define a CBWFQ or LLQ on the physical interface while shaping the traffic on the subinterfaces.

Thanks again:



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