[fusl]

As far as my understanding goes, RFC3021 is unsupported on some router devices widely used as it doesn't have a broadcast and network address and is therefore invalid (/s), so some ISPs default to /30 instead for peering and don't filter out their peering networks in BGP.

[kbirkeland]

I think the reason for using /30s instead of /31s is mostly legacy. It's a 19 year old standard and most vendors support it.

Regardless of the point-to-point subnet used for the local peering connection, it's interesting that that many /30s have leaked into the default-free zone. BCP 194 recommends filtering IPv4 prefixes longer than a /24.