[101996] in North American Network Operators' Group
Re: Cost per prefix [was: request for help w/ ATT and terminology]
daemon@ATHENA.MIT.EDU (Patrick W. Gilmore)
Tue Jan 22 06:39:38 2008
Cc: "Patrick W. Gilmore" <patrick@ianai.net>
From: "Patrick W. Gilmore" <patrick@ianai.net>
To: nanog@merit.edu
In-Reply-To: <319840.19999.qm@web31815.mail.mud.yahoo.com>
Date: Tue, 22 Jan 2008 06:36:05 -0500
Errors-To: owner-nanog@merit.edu
On Jan 21, 2008, at 6:14 PM, David Barak wrote:
> Wouldn't a reasonable approach be to take the sum of a 6500/
> msfc2 and a 2851, and assume that the routing computation could be
> offloaded?
>
> The difficulty I have with this discussion is that the cost per
> prefix is zero until you need to change eigenstate, where
> there's a big cost, and then it goes back to zero again.
>
> Because this isn't really all that new a problem, most vendors
> try not to make devices which have no headroom at all - so kit in
> the lower category seems to be qualitatively different.
We have a winner.
The problem with William's calculation is that he is claiming the
_only_ difference between X & Y is "prefix count". (He said this,
more than once.)
He is dead wrong.
I cannot think of a pair of boxes where one can support a full table
and one can't where the _only_ difference is prefix count. (I am
excluding things like "Box X with 128MB RAM and Box X with 1 GB
RAM".) Even the Sup2 & 3blx have far more differences than just
prefix count. If you do not understand why, you clearly aren't
competent to post to this thread.
For every network, there is equipment that will work, and equipment
that will not. At the top end for very large networks, buying less
than a CRS1 / T640 is simply not an option. And the amount of
engineering going into those boxes to deal with a 1M BGP entry table
is essentially _zero_. Many networks buying those boxes have 100s of
1000s of prefixes in their _IGP_, so the FIB / processor / RAM / etc.
all have to deal.
Near the bottom end, for networks that could get away with a 3750 if
they only supported a full table (which I submit is a pretty small
fraction of the total boxes sold), they can still buy the 3750 and
default to a pair of cheap 2/3/4-port boxes with a gig of RAM and use
those for external connectivity. The problem is perfectly solvable
without jumping to the 7600/3blx.
In between there are more variations than everyone here combined could
possibly imagine. And the requirements are _NOT_ all about prefix
count. Which means you have no basis for comparison. If you try to
force a general comparison, you will be wrong.
So stop asking "what box would you compare?" The answer is "whatever
I need!", not what YOU think is correct, since you are invariably
wrong unless you know everything about MY network.
Anyone who thinks differently is either confused or has an agenda they
are pushing. Or, possibly, trying to sell you a bigger box.
PLEASE, let the thread die.
--
TTFN,
patrick
