why not AS number based prefixes aggregation

Paul Francis francis at cs.cornell.edu
Tue Sep 9 13:02:57 CDT 2008

Sorry, my question was not clear.  By "entries" I meant "routes" or
"prefixes".  For instance, some ISPs today deaggregate in order to
load-balance, so they advertise multiple prefixes or routes instead of one.
Of course, the "right" number would vary from ISP to ISP (as someone already
pointed out to me), but I'm not even sure what the criteria would be for how
many routes one needs to load balance...i.e. depends on the number of AS
neighbors?, depends on the number of depends on the number of BGP neighbors?,
depends on your load balancing mechanism (MEDs versus path prepending versus

The point is this...BGP seems to give use two tools...a machete (AS numbers)
and a scalpel (prefixes).  If I want to cut a steak (load balance), the
machete is too coarse, the scalpel is too fine.  What's the right tool???


> -----Original Message-----
> From: Jean-François Mezei [mailto:jfmezei at vaxination.ca]
> Sent: Monday, September 08, 2008 10:21 PM
> To: nanog at nanog.org
> Subject: Re: why not AS number based prefixes aggregation
> Paul Francis wrote:
> > AS, or even dozens.  So I'm curious...if we could wave a magic wand
> and
> > control the exact number of entries any AS needs to advertise, what
> would
> > folks consider to be roughly the right number of entries?
> Wouldn't this greatly depend on the span/breath of your network ? If
> you
> are a large nationwide (or even international) ISP/network, then you
> want to be able to distribute your network so that someone on west
> coast
> trying to reach one of your west coast IP addresses will have a pretty
> direct route into your west coast infrastructure instead of funnelling
> all traffic into one central location.
> But a smaller ISP based in only one city would not need to distribute
> traffic through different entry points since traffic from each transit
> provider would end up on the same router.
> So I am not sure one could draw any "right number of entries".

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