[RRG] Re: Geographic aggregation-based routing is at odds with reality

["William Herrin" <bill@herrin.us>]

On Thu, Jul 17, 2008 at 9:37 PM, Robin Whittle <rw@firstpr.com.au> wrote:
> I think that geographical-based addressing arrangements, such as
> those proposed in the recent thread by Heiner Hummel and Iljitsch
> ban Beijnum) are not worth considering further because:

Hi Robin,

Let's be cautious about taking an absolute position against geographic
aggregation. Let me relate to you an experience I had a year or so
ago:

At the Democratic National Committee, our upstreams were handled by a
pair of Cisco 6503 switches with Supervisor 2 cards. We were an
origin-only AS, a leaf node in the BGP system.

Here's an important fact about the Sup2 card: it implements the FIB
with a TCAM capable of holding 244k entries. What size did the global
BGP table pass last year? You guessed it. And what happens when the
TCAM fills up? The least specific routes (starting with 0.0.0.0/0)
fall out of the TCAM and have to be process-switched by the main CPU
using a radix tree lookup. So the box with multiple gigabits of
hardware-switching capacity gets very slow very quickly.

My solution to the problem was, in fact, geographic aggregation.
Optimal routing to the pacific rim is not a priority for the DNC. The
target audience is primarily domestic. So I looked up the /8's for
APNIC and squashed them into a few large routes that preferred first
one path and then another if a few "trigger" routes I defined should
happen to drop out. Brought my FIB down to about 180k entries, well
inside the Sup2's limit.

If IOS supported performing this sort of calculation automatically
when it builds the FIB from the RIB, that approach could accommodate
many origin-only ASes like the DNC almost indefinitely. Doesn't
particularly help the core with the two prefixes the DNC announces
though.


All of that having been said, Heiner's approach runs counter to the
economic demand and Iljitsch has his work cut out for him if he wants
to try to demonstrate that additional geographic-centric address
assignment doesn't chase diminishing returns.

Regards,
Bill Herrin



-- 
William D. Herrin ................ herrin@dirtside.com bill@herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
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