[AusNOG] IPv6 Router Advertisements - what they really are doing

[Matthew Walster]

On Mon, 10 Aug 2026 at 05:29, Mark Smith <markzzzsmith at gmail.com> wrote:

> I think a field in IPv6 RAs needs to be renamed, however when I've thought
> about it more, even Router Advertisements should be renamed, because they
> do more than just advertise routers.
>
> That's not realistic now given how prevalent "Router Advertisement"
> terminology is. However, in the intro I've discussed what RAs are really
> doing and how it may be worth remembering to think about what they're
> actually doing.
>

>From https://www.rfc-editor.org/info/rfc4861/#section-3:

     Router Advertisement: Routers advertise their presence together
>                 with various link and Internet parameters either
>                 periodically, or in response to a Router Solicitation
>                 message.  Router Advertisements contain prefixes that
>                 are used for determining whether another address shares
>                 the same link (on-link determination) and/or address
>                 configuration, a suggested hop limit value, etc.


It's not advertising routers, it's a router advertising its capability of
reaching particular things and the parameters needed to use it -- you're
under no obligation to use it, it's just there if you want it. That's how
I've always understood it, anyway. That's backed up by
https://www.rfc-editor.org/info/rfc4861/#section-4.2 because (unlike DHCP,
which is a service) it's advertising capability, not authority.

You somewhat cover that in your draft:

Given the far more general purpose of IPv6 RAs, the name Router
> Advertisements itself is inaccurate. They not only announce a router to
> nodes on a link, they also announce a number of other link and subnet
> specific IPv6 parameters.


Indeed, they're saying "if you use me, you'll need to understand my MTU,
and where I can reach" -- otherwise they would be "routing advertisements"
not "router advertisements".

M
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