/27 the new /24

William Herrin bill at herrin.us
Fri Oct 2 18:09:16 UTC 2015

On Fri, Oct 2, 2015 at 11:50 AM, Mike Hammett <nanog at ics-il.net> wrote:
> How many routers out there have this limitation? A $100 router
> I bought ten years ago could manage many full tables. If
> someone's network can't match that today, should I really have
> any pity for them?

Hi Mike,

The technology doesn't work the way you think it does. Or more
precisely, it only works the way you think it does on small (cheap)
end-user routers. Those routers do everything in software on a
general-purpose CPU using radix tries for the forwarding table (FIB).
They don't have to (and can't) handle both high data rates and large
routing tables at the same time.

For a better understanding how the big iron works, check out
https://www.pagiamtzis.com/cam/camintro/ . You'll occasionally see
folks here talk about TCAM. This stands for Ternary Content
Addressable Memory. It's a special circuit, different from DRAM and
SRAM, used by most (but not all) big iron routers. The TCAM permits an
O(1) route lookup instead of an O(log n) lookup. The architectural
differences which balloon from there move the router cost from your
$100 router into the hundreds of thousands of dollars.

Your BGP advertisement doesn't just have to be carried on your $100
router. It also has to be carried on the half-million-dollar routers.
That makes it expensive.

Though out of date, this paper should help you better understand the
systemic cost of a BGP route advertisement:

Bill Herrin

William Herrin ................ herrin at dirtside.com  bill at herrin.us
Owner, Dirtside Systems ......... Web: <http://www.dirtside.com/>

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