Hello everyone and thanks in advance for your responses.

I just need a sanity check on a problem I've encountered at work. I have three OpenSUSE machines on the same VLAN. One is a multicast server and the other two are supposed to be clients. Since the application is not working, I ran tcpdump on the machines in question and found that the server is indeed multicasting while the clients are not receiving any multicast packets from the application server. My network team swears that it isn't their problem and that I must have something mis-configured. This was previously working, but isn't now. I never had the occasion to run tcpdump before to test this (while it was working) so my observations may be lacking. I do not have any iptables firewall rules enabled.

On the workstations, I do see multicast messages from my cisco switch (OSPF hello messages, CDP, etc). I also see these same messages on the server (along with all the traffic the application is sending). This seemed to confirm to me that tcpdump was working correctly to intercept the packets.

My questions are these:
1. Is there anything on the server that would prevent the multicast packets from going onto the LAN after I see them with tcpdump?
2. Is there anything on the workstations that would be intercepting the multicast packets before I get to see them with tcpdump?

Thanks again for any comments/troubleshooting ideas.

What LAN switches are you using? If they are recent intelligent switches running something like cisco's IGMP Snooping, then they will block any multicast group above 224.0.0.255 that does not have members registered via IGMP.
Anything 224.0.0.x is not allowed to be blocked by a switch as they are link-layer multicasts used in LAN management so to to do so could kill the network - therefore you will see these when doing a promiscuous packet capture.

To get the switch to forward higher multicast addresses to you, you more than likely need to set up an app (that supports IGMP)to join the group in question.

Cheers

I'm just seeing this and my network team is gone for the day, but we are using very new Cisco switches and my multicast address is above 224.0.0.255. I'll get with them tomorrow and see if they have IGMP snooping enabled. That sounds like a likely explanation for what I'm seeing. I'll post information on what I find.

Thanks.