All things related to multicast

[Dan Pritts <>]

It almost certainly depends on the particular switch(es) you are using
but I'd expect that this is how it works.

The alternative is not to have internet multicast working, or for the
switch to know that nobody else outside your LAN wants this multicast
(which, if you use net-local multicast groups, it might if it is smart).  

but...you say "non-igmp-capable routers" - and you suggest that you
think your router should be sending joins/membership reports.  That is
generally NOT the way things work - the router needs to know about group 
memberships so it can pass them along.  So your router probably *is*
"igmp-capable" - it's just not implementing the IGMP recipient side.
This is appropriate for a router.

if your router is doing no IGMP at all, then QED it is not sending IGMP
queries, and your switch won't forward traffic to it.  But then IGMP
probably won't work at all, unless you tell your switch or some other
device to act as queriers (HP switches can do this).




On Fri, Nov 12, 2004 at 05:22:00PM -0500, Jeff Fitzwater wrote:
> This may be slightly off topic but you guys must know the answer to this.
> 
> 
> I believe I understand how IGMP SNOOPING works and everything I have 
> read and tested tells me that the MULTICAST DATA will always be passed 
> toward the QUERIER (This should be the router in most cases).    Is this 
> correct and works as designed so it can handle non-IGMP capable routers 
> that do not generate JOINS (REPORTS).
> 
> In our case we have the multicast SRC and CLIENT downstream of the 
> router but the router is the QUERIER, so therefor the downstream switch 
> hears the report from the client and adds it to his table.  Now when the 
> multicast data flows it not only goes out to the client but also out 
> toward the QUERIER (router).
> 
> I would think this would just suck up unnecessary BW on the router link.
> 
> 
> Q.   Does the data always get sent to the QUERIER even if there are no 
> listeners at the router ???
> 
> 
> 
> I hope I described it OK.
> 
> 
> Thanks for any response.
> 
> 
> 
> Jeff Fitzwater
> OIT Network Systems
> Princeton University


danno
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