[openib-general] IPv6 All Router Multicast Group

[Hal Rosenstock]

Roland Dreier wrote:
>     Hal> So maybe IPv6 unicast routers don't need to receive on this
>     Hal> group (hosts definitely do).
> 
> I think unicast routers would need to listen to the all-routers
> group.  

Do they need to know the other routers ? Isn't that what the 
routing protocols (RIP, OSPF, ...) are about ?

> However, since router advertisements are sent to the all-nodes
> group, a typical IPv6 end node does not need to listen to the
> all-routers group (which is why the kernel doesn't join the
> all-routers group by default).

So an end node doesn't find available routers via this group ?
What you are saying is consistent with the observations for this and the 
router would create the group and there is no one to send to if the
group isn't there (send only join).

>     Hal> For IGMP, I think both hosts and routers need to both send
>     Hal> and receive yet we see a send only join for this group.
> 
> OK, not sure how IGMP works on the host side.

A host needs to tell the multicast router when it is joining and leaving 
a group so the router knows when to join the multicast tree for that group
or prune the tree.
 
>     Hal> The same thing appears to be occuring in these IPv4 (all
>     Hal> routers and IGMP).
> 
> The only IPv4 group I see my systems joining is the broadcast group --
> maybe you have more config options turned on than I do; I'm running
> 
> # CONFIG_IP_ADVANCED_ROUTER is not set
> # CONFIG_IP_MROUTE is not set

I have the same for those options with also CONFIG_IP_MULTICAST = y

-- Hal