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

Patrick W. Gilmore patrick at ianai.net
Sun Jan 20 18:11:48 UTC 2008


On Jan 20, 2008, at 12:22 PM, William Herrin wrote:

>> I think you mean in tiny fractions of a single cent per router per
>> year
>
> No, I don't. The lower bound for that particular portion of the cost
> analysis is easy to calculate:

Your calculation is in error.


> Entry level DFZ router: $40,000
> Stacked 1U layer-3 switches with similar switching capacity and port
> density: $10,000

One of us is very confused.  Given that I order "entry level DFZ  
routers" all the time far less than $40K every month, I am going to  
guess it is not me.


> Difference between the two: The switch stack can't handle the DFZ  
> prefix count.

The difference is much, much, much greater than that.  Can the switch  
do ACLs?  Policy routing?  SFlow with the same sampling rate?  Same  
number of BGP session?  Etc.  In short, if the table were 50K prefixes  
instead of 250K, would these pieces of equipment be equivalent?  The  
answer is a blatant "no".

I can cook stats as well as the next guy, but I try to be painfully  
obvious about my bias.

Given that you say the thread doesn't include more than the number  
here, I don't even think I need to read the thread.  The cost  
(billions of dollars per year) is clearly in error.

-- 
TTFN,
patrick


> Cost delta (attributable to the DFZ prefix count): $30,000
> Expected lifespan in the DFZ of an entry-level router: 3 years
> Prefixes in the table: 245,000
>
> Calculation: The LOWER BOUND for the cost per prefix per router can be
> calculated as:
> ( [entry level router's cost attributable to prefixes]/[expected
> lifespan] ) / [DFZ prefix count]
> ($30,000/3)/245,000 = $0.04 per router per year, i.e. 4 cents.
>
> Bear in mind that 4 cents per year is a LOWER BOUND. It costs AT LEAST
> 4 cents per router per year to carry one prefix in one DFZ router.
> There are also routers in the DFZ which cost $500,000 where much more
> than $30,000 is attributable to the prefix count.
>
>
>> .  While there are 27K ASes ($0.30/year/AS, remember?), there are
>> many more routers which carry a full table.
>
> Yes, there are many more routers than ASes. In my original analysis on
> ARIN, I estimated that there were some 200,000 routers carrying a full
> table in the DFZ. The consensus at the time was that the number was
> closer to 150,000 than 200,000. 150,000 times 4 cents yields a LOWER
> BOUND economic impact of $6,000 per prefix per year, $1.5B overall.
>
> Again, that's a lower bound on the estimate. The upper bound is
> perhaps twice that with the highest probability cost around $8,000 per
> prefix per year.
>
>
>> Comparing cisco & Juniper's annual revenue to the cost of a prefix is
>> like comparing Ford & GM's revenue to the cost of bulbs in
>> headlights.  Hell, most of cisco's revenue is not even related to
>> routers doing a full table.
>
> Of course not. However, it makes a good sanity check on the cost
> estimate: If the costs attributable to prefix count sums to more than
> their revenues then there must be an error in the math. My point was
> that the $8000/prefix/year estimate passes the sanity check with
> plenty of room to spare.
>
>
>
>>> The thread started here:
>>> http://lists.arin.net/pipermail/ppml/2007-September/008927.html
>>> It was originally an argument of about the cost of doing PI for  
>>> IPv6,
>>> which according to Cisco product literature consumes twice the  
>>> amount
>>> of space in the FIB as routes for IPv4.
>>
>> Anyway, thanks for the link.  I must be missing something seriously
>> important to the calculation.  Perhaps it includes things like human
>> time to upgrade equipment or filters or something?  I'll see how the
>> calculation was put together.
>
> Nope, just the numbers you see in the link. I didn't calculate cost
> increases due to manpower,  cost reductions due to equipment reuse
> after its DFZ lifespan, or other factors for which data is sketchy and
> the likely impact to the analysis is small.
>
> Like I said: read the thread, critique the numbers and then add them
> up for yourself. A prefix is surprisingly expensive. If it wasn't,
> folks wouldn't be so concerned about the rising prefix count.
>
> Regards,
> Bill Herrin
>
> -- 
> William D. Herrin                  herrin at dirtside.com  bill at herrin.us
> 3005 Crane Dr.                        Web: <http://bill.herrin.us/>
> Falls Church, VA 22042-3004
>




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