When the BGP table hits 1 million prefixes, will history repeat itself? (posted 2021-03-23)

On the APNIC blog, Danny Pinto asks What will happen when the routing table hits 1024k? Back in 2014, the IPv4 BGP table reached 512k, a common limit in many routers at the time, and some bad things happened. See my post BGP table hitting 512k limit in older routers. And pretty much the same thing happened in 2008, when the BGP table hit 256k.

Interestingly, reaching the 1024k limit was predicted for 2019, but now in March 2019, we're still well below 900k: BGP table growth has leveled off from an average 10% per year in the 2010s to 5% in 2020. My prediction is that we won't reach 1024k IPv4 prefixes in the global BGP table until 2024.

However, asking when we'll reach 1024k IPv4 prefixes is probably asking the wrong question. (But, do read the APNIC blog article for a few relevant pointers.) Back in 2014, there were less than 20k IPv6 prefixes in BGP, and many networks didn't run IPv6 yet. Well, actually, there are 71k networks (autonomous systems) advertising one or more IPv4 prefixes, but only 22k networks announcing one or more IPv6 prefixes.

Still, many networks run IPv4 and IPv6 side-by-side, which means 874k IPv4 prefixes + 114k IPv6 prefixes = 988k total. Add to that internal prefixes, which can add up to many thousands in larger networks, and I'm pretty sure many networks are already over the 1024k limit today, or will be soon.

So I'm sure a few networks will be caught unprepared and run into trouble when the IPv4 table goes over 1024k, but to a much lesser degree than in 2008 and 2014.

In any event, when buying a new router, check if it can handle 2 million total prefixes in the main routing table (RIB) and the forwarding information base (FIB). Also read the fine print, sometimes IPv6 prefixes can count double, and it's relatively common that the 2M limit applies to a combination of firewall rules as well as IPv4 and IPv6 prefixes.

There is usually a separate limit for the BGP RIB, which needs to be much larger, because unlike the main RIB and the FIB, the BGP RIB keeps multiple copies of every prefix: one for each BGP neighbor. So for the BGP RIB you'll want something like 8 million. However, the BGP RIB is usually only limited by available RAM.