Router ID on IPv6-Only

Crist Clark cjc+nanog at
Thu Sep 8 05:38:40 UTC 2022

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.
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