Re: Cost per prefix [was: request for help w/ ATT and terminology]

["Patrick W. Gilmore" <patrick () ianai net>]

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