[rbridge] When would an RBridge say "I don't want layer 2 multicast"?

[Joe Touch]

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Dinesh G Dutt wrote:
> There is a L2 protocol called MMRP (used to be called GMRP) that allows
> me to do the equivalent of IGMP but for L2-only multicast (actually it
> can also be used for IP multicast). So, it should be possible for an
> Rbridge to say "don't send me any L2 multicast that isn't derived from
> IP multicast".

I don't see why these two statements are related. If MMRP can also be
used for IP multicast or not, then how would an operator ever know when
to set this bit to "off"?

I'm suggesting that if we can't give clear advice to an operator on when
to set each value of a bit, we should not include it.

Joe

> James Carlson wrote:
>  
>>>> Joe Touch writes:
>>>>    
>>>>> Donald Eastlake wrote:
>>>>>      
>>>>>> This was put
>>>>>> through a consensus call on the working group mailing list resulting
>>>>>> in the formal consensus determination here:
>>>>>> http://www.postel.org/pipermail/rbridge/2007-September/002470.html.
>>>>>>         
>>>>> Besides your mail, there was only one post from James Carlson endorsing
>>>>> the idea:
>>>>> http://www.postel.org/pipermail/rbridge/2007-July/002400.html
>>>>>       
>>>> Just to make clear (which itself might be impossible at this point):
>>>> the reason I supported it was for symmetry with the other multicast-
>>>> optimizing bits already defined.  If the implementation has some
>>>> reason to know that it has useful local information about non-IP
>>>> multicasts in use (e.g., the subnet in question runs only IP or
>>>> perhaps is known to use GMRP for all multicast addresses), then it can
>>>> set or reset the flag as needed.  If it doesn't (or can't) know about
>>>> non-IP multicast usage, then it should set it to 1 with the rest of
>>>> those who aren't snooping the multicast control protocols.
>>>>
>>>> I somewhat doubt it's going to see much use, but it's also fairly
>>>> cheap -- as long as we already have to support IPv4 and IPv6 control
>>>> bits.  (And since, if you're lazy, you can just ignore it and let the
>>>> downstream discard the unwanted packets.)
>>>>     
> 
> I'm seeing a "something operators can set as desired", but not a reason
> they would ever want to set it. I particularly dislike the idea of
> filtering multicasts based on upper layer info (i.e., whether it's IP or
> not). IGMP is an optimization, but it seems like this bit could break
> things when its use wasn't needed - and I still don't see a clear need.
> 
> Joe
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