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

Jay Hennigan jay at
Mon Oct 10 15:39:06 UTC 2022

On 10/10/22 07:58, 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 ?

Are you multi-homed? If not you can simply take a default.

If so, a better approach might be to apply a max AS rule and take full 
tables plus a default from both (all). Something like "bgp maxas-limit 
4" will optimize routing down to /24 but drop routes with long AS paths 
and punt to default, reducing your table size at the cost of sub-optimal 
routing to destinations that are going to take a convoluted path anyway.

Jay Hennigan - jay at
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV

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