Re: IPv6 certification - IPv6 Router Advertisement Lifetime 0 and Reachable time 10 seconds

[Ted Lemon <[e-mail]>]

On Jan 25, 2021, at 2:30 PM, Nick Hilliard <[e-mail]> wrote:
> I've read the link you cited, but maybe am missing something here.  Can you be clearer about situations where Router Lifetime = 0 means something other than "do not install this router address in the routing table”?

It doesn’t mean that. It means “do not install a default route pointing to this router.”

An example of where this is used in practice is with stub routers, where we want to publish reachability to a stub network prefix without claiming to be able to route to the internet. So we send an RA with a router lifetime of zero and with a Route Information Option for the prefix in use on the stub network.

Someone else earlier mentioned the example of where you want to provide a prefix for use on the local link, but do not want to claim the default route, because you aren’t presently able to route packets to the internet. This can be useful in a small subnetted network where a routing protocol like babel is operating, but there’s no egress route to the internet. In this case, you still want to advertise reachability, but you don’t want to claim the default route; some other router on the link may in fact have reachability to the internet, and by claiming the default route you would be breaking that connectivity.