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

Jason Schiller schiller at uu.net
Wed Oct 31 18:33:56 EDT 2007

On Wed, 31 Oct 2007, Scott Leibrand wrote:

> > 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.
> > 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.
I am not authorized to share specific numbers.  However, currently the
Internet table is 241,847  A large ISP has between 50-150K internal
routes.  In the not so distant past, UUNET had more internal routes than
Internet routes, but some agressive renumbering of our dial-up customers
helped quite a bit.  In short the number of Internal more specifics is
very significant.  

If my customers want shortest path forwarding (as determined by BGP path 
selection) to destinations on the Internet than I need to carry are the
multi-homed more specifics of all of my direct Peers.  That will not
reduce my tables by much.

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