Thousands of hosts on a gigabit LAN, maybe not

Eduardo Schoedler listas at
Sun May 10 00:46:41 UTC 2015

Juniper OCX1100 have 72 ports in 1U.

And you can tune Linux IPv4 neighbor:

Eduardo Schoedler

Em sábado, 9 de maio de 2015, Lamar Owen <lowen at> escreveu:

> On 05/08/2015 02:53 PM, John Levine wrote:
>> ...
>> Most of the traffic will be from one node to another, with
>> considerably less to the outside.  Physical distance shouldn't be a
>> problem since everything's in the same room, maybe the same rack.
>> What's the rule of thumb for number of hosts per switch, cascaded
>> switches vs. routers, and whatever else one needs to design a dense
>> network like this?  TIA
>>  You know, I read this post and immediately thought 'SGI Altix'........
> scalable to 512 CPU's per "system image" and 20 images per cluster (NASA's
> Columbia supercomputer had 10,240 CPUs in that configuration.....twelve
> years ago, using 1.5GHz 64-bit RISC CPUs running Linux.... my, how we've
> come full circle.... (today's equivalent has less power consumption, at
> least....)).  The NUMA technology in those Altix CPU's is a de-facto
> 'memory-area network' and thus can have some interesting topologies.
> Clusters can be made using nodes with at least two NICs in them, and no
> switching.  With four or eight ports you can do some nice mesh topologies.
> This wouldn't be L2 bridging, either, but a L3 mesh could be made that
> could be rather efficient, with no switches, as long as you have at least
> three ports per node, and you can do something reasonably efficient with a
> switch or two and some chains of nodes, with two NICs per node.  L3 keeps
> the broadcast domain size small, and broadcast overhead becomes small.
> If you only have one NIC per node, well, time to get some seriously
> high-density switches..... but even then how many nodes are going to be per
> 42U rack?  A top-of-rack switch may only need 192 ports, and that's only
> 4U, with 1U 48 port switches. 8U you can do 384 ports, and three racks will
> do a bit over 1,000.  Octopus cables going from an RJ21 to 8P8C modular are
> available, so you could use high-density blades; Cisco claims you could do
> 576 10/100/1000 ports in a 13-slot 6500.  That's half the rack space for
> the switching.  If 10/100 is enough, you could do 12 of the WS-X6196-21AF
> cards (or the RJ-45 'two-ports-per-plug' WS-X6148X2-45AF) and get in theory
> 1,152 ports in a 6513 (one SUP; drop 96 ports from that to get a redundant
> SUP).
> Looking at another post in the thread, these moonshot rigs sound
> interesting.... 45 server blades in 4.3U.  4.3U?!?!?  Heh, some custom
> rails, I guess, to get ten in 47U.  They claim a quad-server blade, so
> 1,800 servers (with networking) in a 47U rack.  Yow.  Cost of several
> hundred thousand dollars for that setup.
> The effective limit on subnet size would be of course broadcast overhead;
> 1,000 nodes on a /22 would likely be painfully slow due to broadcast
> overhead alone.

Eduardo Schoedler

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