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

Raymond Burkholder ray at
Mon Oct 10 15:49:47 UTC 2022

On 2022-10-10 09:39, Jay Hennigan wrote:
> On 10/10/22 07:58, Edvinas Kairys 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.
> 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.

And run something like netflow to determine high traffic AS paths, and 
optimize those into your filtering.

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