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

Jim Troutman jamesltroutman at
Mon Oct 10 15:35:43 UTC 2022

If you filter out /23 or longer you cut the v4 table size about in half.  I
have done this with some edge and eyeball network clients that had really
old or underpowered routing gear and upgrades were just not in the budget,
and they could barely spell BGP.

I know of a number of ASNs with SUP720 era gear still in production this
way in 2022 (the power bill is usually someone else’s budget!).

Be sure to take default from a couple upstreams and allow /24s for the
peers on your IXP links that matter (CDN, etc) and your traffic is mostly
fine.  Maybe not always taking the most direct return path, but it gets

Inbound traffic distribution isn’t affected and that is all most eyeball
networks care about.

On Mon, Oct 10, 2022 at 11:26 AM Nick Suan via NANOG <nanog at>

> There's 69,055 pure /24's allocated or assigned directly from an RIRs. At
> least c,d,e, and g root servers only have /24s allocated to them. Major
> services like Cloudflare only advertise the /24 without advertising an
> aggregate.
> Unless you're also getting a default from upstream, it sounds like you're
> going to end up wasting the money you saved on chasing down subtle
> brokenness.
> On Mon, Oct 10, 2022, at 9:58 AM, Edvinas Kairys wrote:
> Hello,
> We're considering to buy some Cisco boxes - NCS-55A1-24H. That box has
> 24x100G, but only 2.2mln route (FIB) memory entries. In a near future it
> will be not enough - so we're thinking to deny all /24s to save the memory.
> What do you think about that approach - I know it could provide some
> misbehavior. But theoretically every filtered /24 could be routed via
> smaller prefix /23 /22 /21 or etc. But of course it could be a situation
> when denied /24 will not be covered by any smaller prefix.
> What do you think about this approach ?
> Also maybe you know - some advices for edge routers that have at least
> 8x100G interfaces and "good" memory for prefix count ? Thanks
> --
Jim Troutman,
jamesltroutman at
Pronouns: he/him/his
207-514-5676 (cell)
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