[c-nsp] shaping w/sub interfaces - drops

Jay Hennigan jay at west.net
Wed Dec 21 14:26:14 EST 2011

On 12/21/11 11:11 AM, Dan Letkeman wrote:
> Hello,
> I'm wondering if its possible to eliminate drops using shaping?  I
> have a sub interface set-up for guest access and I want to limit all
> access to 3mbps and http access to 2mbps.  If I apply a policy to the
> sub interface I continuously see drops on the http class when it runs
> in and around 2mbps.  Its just web browsing so I don't ever want to
> drop the packets just retransmit.

When you limit traffic by any means you may have the choice to either
delay the excess packets or drop them.  Delaying the packets means
storing them in a buffer until the traffic falls below the limit, then
forwarding them.

The buffers have a limited size.  If there is more traffic than the
buffers can hold, it will eventually be dropped.  There is lots of
discussion and several examples regarding this with "leaky bucket"

So if there is more traffic than the configured shape rate (or more
traffic than the physical medium can handle) it will get dropped either
immediately or when the buffers fill up depending on configuration,
amount of memory, etc.

Upper-layer protocols such as TCP can mitigate this by slowing the input
rate when drops are detected.  But if there is more traffic coming in
than the buffers, shape limit, or outbound medium can handle, it must
get dropped.  There's nowhere else for it to go.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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