maximum ipv4 bgp prefix length of /24 ?

Mon Oct 9 19:21:31 UTC 2023

> On 10 Oct 2023, at 5:35 am, Delong.com <owen at delong.com> wrote:
> 
>> Now I’m trying to understand what your grimmer story for IPv4 might be here Owen. Since 2005 the number of IPv4 FIB entries per origin AS has increased fropm 8 to 12 in the past 20 years - or a 50% increase. Over ther same period the number of IPv6  prefix advertisements per origin AS has increased from 1.5 to 6, or a fourfold increase. If anything, the IPv6 story appears to me to be a far greater cause for concern, but you may have a different interpretation of this data.
> 
> I admit I’m surprised that IPv6 has gotten to an average of 6, but it would be interesting to know why that is. Honestly, I expected it would end up closer to 4. I wonder how much of that is PI stub networks being originated by upstream transit networks. I also wonder to what extent the “average” might be misleading here. Is it a few ASs with large numbers of prefixes and mostly 1-2 prefixes per AS or is the average representative?
> 
> I know that in IPv4, for example, there are several ASs originating MANY prefixes and lots of smaller ASs originating <4 prefixes.
> 
> My grimmer picture for IPv4 is about the intrinsic pressure to deaggregate that comes from the ever finer splitting of blocks in the transfer market and the ever finer grained dense packing of hosts into prefixes that is forced from address scarcity. Those pressures don’t (or at least shouldn’t) exist for IPv6.
> 

The questions you ask Owen are obviously answerable by anyone with access to a BGP routing table dump (which is pretty much anyone!).

BGP is many things - it is a topology maintenance protocol, but its a traffic engineering protocol and an attack mitigation protocol. In the latter two cases advertising more specifics play a crucial role. The pressure to slice and dice in IPv4 is a mix of reachability in a space where address availability is under acute pressure, and TE and DOS mitigation. The pressures on IPv6 are predominately from the latter two categories. I suspect that as IPv6 becomes a larger part of the traffic mix (and inexorably that appears to be happening) then the TE and DOS issues become more of an operational concern, hence rising more specifics in IPv6. 