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

Mark Smith <> Mon, 25 January 2021 22:15 UTC

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From: Mark Smith <>
Date: Tue, 26 Jan 2021 09:15:03 +1100
Message-ID: <>
Subject: Re: IPv6 certification - IPv6 Router Advertisement Lifetime 0 and Reachable time 10 seconds
To: Ted Lemon <>
Cc: Nick Hilliard <>, Alexandre Petrescu <>,
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On Tue, 26 Jan 2021, 06:50 Ted Lemon, <> wrote:

> On Jan 25, 2021, at 2:30 PM, Nick Hilliard <> 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.

I imagine a more general scenario when thinking of  RA RL=0 use case.

One router is the link's default router, likely to the Internet, so RA RL
!= 0.

Another router on the link provides access to a specific subset of
destinations e.g. perhaps over an IPsec VPN.

That router announces RA RL=0, and Route Information Options for those
destinations, so hosts send directly to the subset destination router for
those destinations.

It's not the only way to get this forwarding working, however RA RIOs is
the most efficient for this subset of destinations because hosts of
informed of exactly which router they need to reach them.

> 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.
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