[j-nsp] Spine & leaf

[adamv0025 at netconsultings.com]

> Of David Sinn
> Sent: Wednesday, June 27, 2018 7:55 PM
> 
> 
> > On Jun 27, 2018, at 8:40 AM, Thomas Bellman <bellman at nsc.liu.se> wrote:
> >
> > On 2018-06-26 21:38, David Sinn wrote:
> >
> >> OSPF scales well to many multiples of 1000's of devices.
> >
> > Is that true even for Clos (spine & leaf) networks, and in a single
area?
> 
> Yes for multi-tiered Clos, as that was the original ask and where my
reference
> is coming from.  However, it is not in a single area.  But if you are
doing a
> multi-tiered then the areas can fall out fairly naturally from the
topology.
> 
> > My understanding, solely based on what others have told me, is that
> > the flooding of LSAs in a Clos network can start to overwhelm routers
> > already at a few hundred devices, as each time e.g. a spine sends out
> > an LSA, all of the other spines will hear that from each of the
> > leaves, all more or less simultaneously.  And since the OSPF protocol
> > limits lifetimes of LSAs to 1 hour, you will get a constant stream of
updates.
> 
> Even Quagga/FRR has some measure of skew in the refresh interval to insure
> that the re-advertisements aren't all clumped into one batch of updates
from
> a given node.  And even having a really bad day where you loose power to a
> good number of devices that basically all start up at the same time
doesn't
> cause measure-able congruence of updates.  The skew in type-1's as the
> boxes randomly bring up adjacencies and re-advertise their reachability
gives
> you a good spread across the refreshes over time which generally grows
over
> time.  So while there is a continual background, even a single core PPC
CPU's
> on commodity boxes can handle it with multiple of the available white-box
> OS's.
> 
Haven't you guys heard of DNA LSAs to get rid of the flooding? -this is de
facto standard in SP core networks.
Also if you are having troubles keeping a reasonable number of IGP routes
there's the hiding of transit-only networks RFC 6860 (all you really need
are just the RIDs)? 

adam

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