[ppml] Effects of explosive routing table growth on ISP behavior

Scott Leibrand sleibrand at internap.com
Wed Oct 31 18:01:11 EDT 2007

Jason Schiller wrote:
> When routing slots become scarse we all will have the difficult decesion
> of which routes to install.  
> It is my suspicion that ISPs will discard non-customer routes first before
> customer routes.  The first set of routes likely to be discarded are
> non-customer routes that are a more specific of an aggregate.  This will
> break TE.  
Yes, it will *partially* break TE.  I maintain that the vast majority of 
the TE benefit remains, however, via more-specific announcements to your 
upstreams, localpref communities, and AS path prepends.

> The next set of routes likely to be discarded are non-customer
> aggregates.  
> At this point, a paying customer may complain that it cannot reach some
> non-customer destination.  The ISP will explain that the destination in
> question is not a customer, and does not pay to have its route in the
> table.  The ISP will then offer the option for his or her customer to pay
> to have the route in the table, or the destination can pay to thave their
> route in the ISp's table.  Talk about net neutrality issues.

I think there's a less messy alternative to going this far, which I have 
attempted to outline in other messages & threads.

> On Wed, 31 Oct 2007, Scott Leibrand wrote:
>> I don't think that's true.  Today, anything you advertise in IPv4 down
>> to a /24 will be accepted more or less by everyone.  However, that's
>> only a convenience for optimal TE.  In order to just maintain
>> reachability, all you really need is for your upstreams (all your
>> transit providers and all their transit providers) to accept your
>> deaggregated routes from you and from each other.  The rest of the world
>> need only accept the RIR-assignment-size covering aggregate, route the
>> traffic toward one of your transit providers, and then let the
>> more-specifics take over when they hand the traffic off to them.
> For large ISPs with widespread peeing, and lots of mult-homed customer
> overlap, this basiclly means they need to carry nearly all of the routes.

Perhaps you could help us understand the magnitude of the difference a 
little better with some actual statistics, if you could share them.  Of 
the /23 and /24 routes in your table (excluding class C swamp space), 
how many (and what percentage) are non-customer routes?  My intuition / 
hypothesis would be that the non-customer routes outnumber the customer 
routes, so you would still get significant benefit from filtering the 
non-customer more-specifics if we come to a routing-table-size crunch.


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