Large IPv6 Multicast Domains

Benedikt Stockebrand bs at
Fri Jun 6 11:51:36 CEST 2014

HI Jim and list,

"James Small" <jim.small at> writes:

> Basically something compliant with RFC 4610.
> With IPv4 I would use the approach described in RFC 3446.

OK, now I understand why I didn't understand your initial question.
I assumed with "anycast RP" you were talking about anycast within a
single subnet...

> With IPv6 it seems like the
> replacement is "Anycast-RP using PIM" as described in 4610.  My
> concern was using this approach for a large global backbone, but
> sounds like that's the only option.

OK again; I was thinking about the scale of a large enterprise with
multiple sites connected via VPNs, MPLS, or whatever.

> So essentially I should stop thinking local "domains" and think
> globally for any IPv6 multicast deployment?

I think so, but I'm not sure; so far I admittedly haven't had to deal
with that sort of setup.

> Coming from an IPv4 Multicast background, I'm having a hard time
> accepting that my only Inter-domain RP option is embedded RP.

Well, there are usually two possible explanations for this: Either IPv6
has managed to get rid of some legacy approaches and/or workarounds for
IPv4-specific limitations, or it is still missing some sort of feature.
In this case however I guess it's the first case.

And when it comes to the combination of scopes and embedded RPs, at
least in the environments I've come into contact with, that's a *huge*
advantage over IPv4 multicast routing; especially so if your multicast
routing setup is somewhat fluctuating all the time.

> I understand the issues with MSDP and why it wasn't extended to
> support IPv6.  However, it still feels like there should be some kind
> of inter-domain RP communication/synchronization option.  Perhaps I
> have not fully grokked IPv6 Multicast yet?

Well, who has?  However, since I've only done any multicast routing with
IPv6 really, I find the term or concept of a "domain", or rather that of
"inter-domain multicast routing" somewhat unclear myself.  Unless I
misunderstand RFC 4610, the entire domain concept, or at least the idea
of "inter-domain" is gone; instead, what is called a domain in there is
the "reach" (not an official term) of whatever multicast group, and that
"reach" is preferably defined by a suitably defined/chosen scope that
is then encoded in the multicast group's scope nibble.

But again, I haven't yet dealt with this at an ISP level, really; my
focus was entirely at the enterprise scale.



Benedikt Stockebrand,                   Stepladder IT Training+Consulting

          Business Grade IPv6 --- Consulting, Training, Projects

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