Router ID on IPv6-Only

Jon Lewis jlewis at
Thu Sep 8 19:39:46 UTC 2022

On Wed, 7 Sep 2022, Crist Clark wrote:

> 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.

I have no idea what your v6 numbering plan is, but given ours, I don't see 
how we could get away with using bits 32-63 from any (loopback or 
real) interface v6 address [that we've assigned] as the router id.

As you say, it's just a 32-bit number, and has to be unique within your 
AS.  You could easily abuse some of your v4 space to be tracked in your 
IPAM or some other db as router IDs (and still use those IPs on servers or 
whatever since the routers aren't really "using them" from an IPv4 routing 
perspective).  Or you might pick a suitably sized subnet from to be used as router IDs.  That would avoid the confusion of 
"why is server X's IP the router-id for these routes?"

  Jon Lewis, MCP :)           |  I route
  StackPath, Sr. Neteng       |  therefore you are
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