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

William Herrin bill at
Mon Oct 10 15:26:22 UTC 2022

On Mon, Oct 10, 2022 at 7:58 AM Edvinas Kairys < at> wrote:
> 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 ?

If you have a default route that works and you don't have any
downstream customers which expect a full routing table, this is fine.
You just won't get as good results with the /24s. Beware that MOST
Internet /24 routes are NOT covered by a shorter prefix so unless you
specifically cover them they will be lost. This will severely impact
your Internet connectivity.

The Internet FIB is around 900k IPv4 routes. You have years before
exhausting a 2.2M table.

Bill Herrin

For hire.

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