[j-nsp] what’s the story behind MPC5E

[Adam Vitkovsky]

> Saku Ytti [mailto:saku at ytti.fi]
> Sent: Saturday, May 21, 2016 12:11 PM
>
> On 21 May 2016 at 04:15, Adam Vitkovsky <Adam.Vitkovsky at gamma.co.uk>
> wrote:
>
> Hey,
>
> > So there are no inter XM HSL2 links on the card please?
> > For it to constitute as two PFEs I would then, as per design, expect there to
> be a separate set of VOQs per each XM(PFE) so that each XM can
> backpressure towards ingress LCs independently, is that the case please?
> > Also how does the backpressure work in this setup when the common XL
> gets oversubscribed does it signal backpressure to both XMs or does it know
> which XM is responsible for most of its cycles so it will issue backpressure
> signal only to that XM?
> > If the above is not met then I would consider it as a single PFE with two
> discrete arbiters feeding a common XL (which I think is just asking for
> trouble).
>
> Both XM connect to fabric and manage their own requests/grants, I don't see
> how that's different just because you share XL.
>
That's not the entire truth, yes XM is responsible for requests/grants but it's the XL dictating the pace at which the grants are issued.
And I doubt that the common XL can issue backpressure signal to each XM selectively based on how much traffic is receives from it.

> And I don't share your worry about congestion, if XL gets congested there is
> no QoS to my knowledge, you can't know who gets to use PPE's resource and
> who does not, your box is oversubscribed and you need to fix it.
> I don't understand what kind of benefits you are getting by having dedicated
> XL per XM, are you thinking of putting trash traffic behind one XL, where you
> don't care if it's starved occasionally, because all traffic is equally trash and
> then important traffic behind another XL?
> If so, then I guess from your POV maybe it is single PFE and you'll need
> another linecard to do this sort of planned oversubscription.
>
> I'm very very dubious if there is business case to plan to congest XL.
> I consider lookup congestion fault which needs to be fixed.
>
Well during DDoS attack the XL will get congested so you better be prepared for that.


adam







        Adam Vitkovsky
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