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 :)
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
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 Brandon Martin
> Sent: October 21, 2022 4:30 PM
> To: nanog@nanog.org
> 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