Re: BGP prefix filter list

[William Herrin <bill () herrin us>]

On Mon, May 20, 2019 at 4:09 PM Seth Mattinen <sethm () rollernet us> wrote:
> On 5/20/19 3:05 PM, William Herrin wrote:
> > The technique you describe was one variant of FIB Compression. It got
> > some attention around 8 years ago on the IRTF Routing Research Group and
> > some more attention about 5 years ago when several researchers fleshed
> > out the possible algorithms and projected gains. As I recall they found
> > a 30% to 60% reduction in FIB use depending on which algorithm was
> > chosen, how many peers you had, etc.
>
> A good start would be killing any /24 announcement where a covering
> aggregate exists.

Only when the routes are identical -- same origin, same path. Otherwise
you're potentially throwing away your only path to that destination. And if
you lose the aggregate, the /24 has to be reintroduced to the FIB. Which
means you have to interlink the routes in the RIB data structure so that
the update algorithm dealing with the aggregate knows there's an associated
/24. There's some real subtlety a FIB Compression implementor must take in
to account.

Regards,
Bill Herrin


-- 
William Herrin
bill () herrin us
https://bill.herrin.us/