I did a task based on sockets which is listening on multicast address.
It works when I send a multicast message from PC, and my board answer. So far so good.
But I took a wireshark capture and lwip doesn't send an IGMP join group. Consequence: packet from PC are sent to the entire network/switch. Then there is no difference with a broadcast packet from my opinion.
I check in the igmp code and the program pass through the join group ( igmp_send() ) but nothing ontgoing on the network.
Just remember that IP multicast terminates in an Ethernet broadcast. If your
switch supports IGMP snooping, it will keep the Ethernet broadcast off all
the ports. If the switch never sees the IGMP (or got confused) it probably
will send all Ethernet broadcasts out ALL switch ports. This is a fail-safe
for most smart switches. A cheap switch usually doesn't support IGMP
snooping, so it will broadcast all Ethernet broadcasts out ALL switch ports.
Probably with your testing, the switch got confused. The "good reset" caused
it to begin snooping again, and thus sending the broadcasts only out the
switch ports that sent an IGMP Membership Join. Do you see any IGMP
Membership Queries? Whoever the rendezvous point is should be sending
membership queries. This is to prune any members of the multicast that have
'dropped' off. If this is a small LAN, you probably don't see any of that,
and it is a simple IGMP snooping situation.
From: lwip-users-bounces+bob=[hidden email] [mailto:lwip-users-bounces+bob=[hidden email]] On Behalf Of pcu
Sent: Monday, May 12, 2014 1:57 AM
To: [hidden email] Subject: Re: [lwip-users] multicast and join group issue
Now it works, without in the firmware...
May be after a good reset of switch, pc and router.
And the messages are not more broadcasted, just send to the right devices.