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

Jon Sands fohdeesha at
Thu Oct 27 14:49:14 UTC 2022

Been doing exactly this for a couple ASNs for a few years now with
surprisingly good results (thanks to advice way far back from my good
friend Brandon Martin above, coincidentally). One of them is even on an L3
switch with something like 96k max routes. Taking defaults from two
upstream providers and ECMPing between them. This particular AS is a pretty
predictable network so after running netflow for a while, compiling a list
of the top ~1000 outbound ASs we talk to, then creating route filters to
allow any prefixes from this AS list into our forwarding table, it now has
something like 98% of it's traffic by volume covered by specifics from all
our upstreams, and of course ECMP defaults to fall back on for the
remaining 2%. Not pretty, but have had surprisingly zero issues or traffic
weirdness over a few years now - when customers want to play bgp but refuse
to buy actual routers you have to get creative :)

On Mon, Oct 24, 2022, 11:47 AM Adam Thompson <athompson at> wrote:

> I can't believe that never occurred to me in all the time I was doing
> that, 'way back when...  <facepalm>
> Thanks for pointing that out!
> -Adam
> Adam Thompson
> Consultant, Infrastructure Services
> 100 - 135 Innovation Drive
> Winnipeg, MB R3T 6A8
> (204) 977-6824 or 1-800-430-6404 (MB only)
> Chat with me on Teams: athompson at
> > -----Original Message-----
> > From: NANOG < at> On
> > Behalf Of Brandon Martin
> > Sent: October 21, 2022 4:30 PM
> > To: nanog at
> > Subject: Re: any dangers of filtering every /24 on full internet
> > table to preserve FIB space ?
> >
> > On 10/20/22 17:50, Adam Thompson wrote:
> > > 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.
> >
> > As a note, you can get redundancy (but still none of the best-path
> > advantages of having multiple transits) by asking your transits to
> > originate default in their BGP feed and then selectively accepting
> > it.
> > You can either ECMP it or pick priority with localpref.
> >
> > You need multiple full-view transits for this to work, though.
> >
> > --
> > Brandon Martin
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