Bandwidth Command Versus Shaping Command

[Alex Pfeil]

If we setup a QoS policy based on the bandwidth of a circuit, and use the bandwidth command on the interface to specify the circuit bandwidth,  does that eliminate the need to user QoS Shaping policy?

[paul driver]

Hello

Setting the BW will define the minimum BW to the interface however it wouldnt prohibit congestion and if this is experienced you could be subject to possible packet drops, however is you shape to the specified CIR rate again if you do experience congestion packets will be buffered for transmission thus are less likely to be dropped.

[Alex Pfeil]

After reading your answer, let me ask the question in a different way.

If you place the bandwidth command on an interface as below:

int gi0/0

bandwidth 10000

exit

Then place a QoS policy on the same interface without shaping.

int gi0/0

service-policy output qosExample

exit

And in each class of the policy-map, use the bandwidth percent command.

Would that effectively shape the traffic? The only difference that I can think of is that shaping is going to specifically send the traffic at a specified bandwidth.

[paul driver · Accepted solution]

Hello
Arh now that's different, Setting the BW on the physical interface like you have stated doesn't actually set the actual CIR rate of the interface it will however be use by any dynamic routing protocol to calculate its protocol cost on the interface which inturn can have adverse effect on how your traffic is routed.

However specifying the BW within a QOS policy and applying that policy to the physical interface will define the BW on the link, so apologies for misunderstanding your OP, but again it will not prohibit congestion like shaping can do via a qos policy

[Joseph W. Doherty]

If I remember correctly, an interface policy-map will use the "bandwidth" defined on the interface for its percentage calculations.

That said, policy classes, without an explicitly defined shaper don't shape traffic.

LLQ, though, does have an implicit policer which, at least when it was tested several years ago, only triggered when there was congestion (unlike an explicit policer which is active all the time).

As noted by Paul, what a policy-map class bandwidth statement does is set a minimum guarantee, but depending on actual traffic and how the policy-map is defined, you often get more than what's defined (even with a congested interface).

For example, given:
policy-map sample
class sampleClass
bandwidth percent 10
class class-default
bandwidth percent 10

For the above, sampleClass is guaranteed 50% of the bandwidth.