Re: Choosing WRED Min and Max Thresholds

Hi Greg,

I think the important point in this discussion is that the wred values only
apply to packets that are sitting in the software queue waiting to be
processed. For example, if your link is running @ 10% of its rated speed
i.e 4.4mbps on a DS3 then the packets are going to be sent out on the wire
without any delay and are not subject to software queueing as the circuit
speed is fast enough to serialise the packets straight onto the wire.

On the other hand if you were sending enough data to clog your software
queue wred would kick in (depending on the queue depth). This is where the
min & max thresholds come into play.

Most networks I have worked on run with the default values however certain
scenarios may benefit from custom values, it just depends :)
Cheers,

Matt

On Mon, Nov 1, 2010 at 9:34 PM, Gregory Gombas <ggombas_at_gmail.com> wrote:

> Hi Gang,
>
> I am trying to choose the best WRED Min and Max Thresholds for a DS3.
>
> So far the best guide I could find was here:
>
> http://www.cisco.com/en/US/docs/ios/11_2/feature/guide/wred_gs.html
>
> However, neither the Cisco default values nor the recommended values on
> that
> page make any sense.
>
> For example, the default value for the WRED maximum threshold on a DS3
> interface is 40 packets.
>
> Assuming a maximum packet size of 1500 bytes that would mean the most
> throughput you would get on a DS3 interface with WRED would be 40 packets *
> (1500 bytes per packet) * (8 bits per byte) = 480,000 bps or 480 kbps.
>
> Which means you would be at full drop at only 480 kbps?!?
>
> Even if you took the recommended setting of 367 packets for maximum
> threshold:
>
> 367 packets * (1500 bytes per packet) * (8 bits per byte) = 4,404,000 bps
> or
> 4,404 kbps (1/10 the bandwidth of a DS3)
>
> I know the goal of WRED is to drop packets before you reach congestion, but
> dropping all packets when reaching 1/10th of the DS3 bandwidth seems a
> little ridiculous.
>
> Am I missing something here?
>
> Thanks,
> Gregory Gombas
> CCIE #19649
>
>
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Received on Mon Nov 01 2010 - 22:48:49 ART