During some IPv6 numbering discussions at work today, someone had a question that I hadn't really considered before. How to choose 32-bit router IDs for IPv6-only routers. Quick background. We have a requirement to convert a significant portion of our network to IPv6-only over the next few years. Previously, I, and everyone else on the team, have only ever set up routers in dual-stack environments. Choosing a router ID for use in routing protocols just followed whatever rules you used for your IPv4 networking. You used the same router ID in IPv4 and IPv6. Well, now there is no IPv4. But BGP, OSPFv3, and other routing protocols still use 32-bit router IDs for IPv6. On the one hand, there are plenty of 32-bit numbers to use. Generally speaking, router IDs just need to be unique inside of an AS to do their job, but (a) for humans or automation to generate them and (b) to easily recognize them, it's convenient to have some algorithm or methodology for assigning them. Has anyone thought about this or have a good way to do it? We had ideas like use bits 32-63 from an interface. Seems like it could work, but also could totally break down if we're using >64-bit prefixes for things like router-to-router links or pulling router loopbacks out of a common /64. Also, various network OS implementations will typically automatically choose a router ID from the IPv4 addresses on the router by some algorithm (e.g. numerically lowest) if not explicitly configured. Was curious what IPv6-only routers do. Haven't had the chance to get on some lab gear or GNS3 to just try it and see.