RE: any dangers of filtering every /24 on full internet table to preserve FIB space ?

[Adam Thompson <athompson () merlin mb ca>]

I can't find the original message, so replying to the wrong spot in the thread, but... no, filtering /24s is a bad idea 
if you want (more or less) all your packets to get to their destinations.

If you filter all /24s you will lose reachability to 4x /24s I publish that have no covering route because they are not 
contiguous and not part of any larger logical aggregate.  Then there's the 10-20 legacy /24s I *don't* currently 
publish - if I start advertising them, you won't be able to reach them, either, because they're in the same boat: 
discontiguous singletons.  There are a LOT of legacy discontiguous IPv4 singletons assigned out of the old Class-C 
space to small/medium businesses, schools, etc. in the pre-ARIN days, and I would guess that the vast majority of them 
do not have a correct covering /23 or larger - certainly none of the ones I'm currently working with/aware of do.

I believe there's at least a couple of DNS servers running in my /24s, so you could potentially lose access to much 
more than those /24s.

Your packet will *probably* hit a next-hop carrier who happens to have the more-specific /24, and it will *probably* 
eventually reach me, but I thought everyone more-or-less agreed that internet router was already nondeterministic 
enough as it is?
IMHO, if you don't want all the /24s in your FIB (or even RIB!), just pick a carrier, set a default route, and stop 
worrying about all the headaches BGP provides.
Alternately, a valid technique is to have a default route AND a partial BGP feed (a filtered full feed is by definition 
a partial feed).  That helps optimize outbound routing a little bit, you still get the advantage - mostly - of multiple 
inbound carriers; but you still have to pick one carrier to do the heavy lifting for you.  And you are paying them to 
route for you, so that's not an unfair shifting of the routing burden, unlike relying on covering routes.  Note that 
this approach does NOT provide any redundancy, unlike having full BGP feeds.

Separately, I don't know if Geoff has produced such a survey/article, but if not he can probably type it from memory by 
now :-).

Adam Thompson
Consultant, Infrastructure Services
MERLIN
100 - 135 Innovation Drive
Winnipeg, MB R3T 6A8
(204) 977-6824 or 1-800-430-6404 (MB only)
https://www.merlin.mb.ca
Chat with me on Teams: athompson () merlin mb ca

> -----Original Message-----
> From: NANOG <nanog-bounces+athompson=merlin.mb.ca () nanog org> On Behalf Of
> Stephane Bortzmeyer
> Sent: October 10, 2022 10:21 AM
> To: Edvinas Kairys <edvinas.email () gmail com>
> Cc: NANOG Operators' Group <nanog () nanog org>
> Subject: Re: any dangers of filtering every /24 on full internet table to
> preserve FIB space ?
>
> On Mon, Oct 10, 2022 at 05:58:45PM +0300,
>  Edvinas Kairys <edvinas.email () gmail com> wrote
>  a message of 35 lines which said:
>
> > But theoretically every filtered /24 could be routed via smaller
> > prefix /23 /22 /21 or etc.
>
> I don't think this is true, even in theory, specially for legacy
> prefixes. There is probably somewhere a Geoff Huston survey on /24
> without a covering route.