Re: [DLC]Multicast

On Mon, Mar 15, 2010 at 01:58:46PM -0500, Matthew P Kemp wrote:
>   Typically when you look at multicast you are looking at the routers
> (unless the switch is handling IGMP), the switch typically treat
> multi-cast traffic as if were a broadcast.  The Ethernet card is just

Depending on the switch, it may support something generically called
IGMP snooping, which means the switch will try IP multicast packets
a bit differently than broadcast.  With snooping, the switch will
forward group messages only to ports that have expressly indicated
interest in the group with a IGMP message.  Cisco switches also have
something called CGMP.  There has been a history of potential issues
with IGMP snooping.  Basically anytime you start forcing your so-called
L2 device to start examining L3 or higher info, things get tricky.  For
example, there was a problem with Cisco switches flooding multicast
(basically like a broadcast storm) whenever there was a spanning tree
topology change.  In large-use streaming multicast environments, that
would cause all sorts of reliability and performance issues.

John
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