All things related to multicast

[Dan Pritts <>]

One good thing about IGMPv3 today:

Its messages go on their own multicast group, rather on the group that
they control.  This makes the job of an IGMP-sniffing ethernet switch
MUCH simpler.  It just needs to watch traffic on the IGMP group, rather
than processing ALL multicast traffic to look for the IGMP messages.

> However, RFC 3376 mandates that
> 
>    In order to be compatible with older version routers, IGMPv3 hosts
>    MUST operate in version 1 and version 2 compatibility modes.  IGMPv3
>    hosts MUST keep state per local interface regarding the compatibility
>    mode of each attached network
> 
> So, old boxes running ASM will cause new boxes running SSM to fail, at 
> least on the
> local LAN.

I don't quite understand what you're getting at.  

Maybe I'm missing something, but my understanding is that the router
needs to accept v1 & v2 IGMP messages, and that hosts must be able to
talk to old routers, but there's nothing that says that an IGMPv2 host
makes the entire LAN unable to do SSM.

danno