3348 igmp snooping is not working

As I understand it you only configure IGMP snooping command from the "global scope".  Is that correct?  If so then that's not quite enough.  For the 3348, there are several other commands you must configure in order for it to work.  Please read the document carefully and make sure you configure the switch correctly.  It is not enough to just enable IGMP snooping from the Global scope on the 3348 switch.

If you have more questions please send me your configuration (do a "show running-config" then send me the configuration on your switch).

Cuong.

Although there are really no standard for how IGMP snooping works in L2 switches, this is a very typical behavior for L2 switches when implementing this feature.  The L2 switch normal behavior is to flood unrecognized packets of which multicast and broadcast are considered to be and so the default behavior is to flood.  Remember that an L2 switch primary responsibility is to switch Ethernet packets.  The support for IGMP snooping in the L2 switch is really meant as a helper function and is there only to support multicast routers and media stream servers. 

So let me explain the idea of IGMP snooping for L2 switches.  Note the word "snooping".  Normally the L2 switch does not really have a role in this typically L3 function.  A media stream is generated from a server to a multicast address.  The multicast router will forward traffic to a set of ports on which it has heard JOIN requests.  The client may join at anytime even prior to the stream initiating.  The router will not send out traffic to ports on which no JOIN has been heard.

So what you described previously is actually a function of how L3 switches (multicast routers) work not how L2 switches work.

So officially an L2 switch has no role in this operation (no standard defined role).  But if the L2 switch remain passive and behave normally where it simply flood unrecognized packets then it will result in unnecessary traffic going to ports on which no client has joined.  So the L2 switch helps by "snooping" the IGMP packets it sees.  By looking at the IGMP packets it can determine where the multicast routers are and the ports on which it has seen an IGMP join for clients of a multicast group.  Knowning where the multicast routers are will help the switch to forward IGMP report (join request) for a multicast group only to the router (necessary to prevent accidentally suppressing JOIN request from clients for the same multicast group when using IGMPv1 or v2 protocol).  Knowning which ports have clients for a multicast group help the L2 switch to know where to forward MAC multicast packets.

This is the extent of IGMP snooping function in the L2 switch, which is the typical behavior for L2 switches in regard to IGMP snooping feature.

Let me ask you something here.  Is your network setup like this?

• media stream server connected to the L2 switch.
• multicast router connected to the L2 switch.
• media stream clients connected to the L2 switch.

In this case the media stream server is sending out media stream on a multicast address to the L2 switch.  This is flooded.  The multicast router see the multicast stream and periodically sends out a query for membership.  If no membership JOIN is seen then the multicast router will not send the media stream, but in the meanwhile the media server will continue to send multicast packets to the L2 switch which will continue to flood.  When a client JOIN then the switch will know where the clients are and stop flooding.  The MAC multicast will be sent only to the router and to the client.  Note that the router is actually also repeating the multicast packets but it will not send back out the port on which it received it so you do not see duplicate multicast packets on the network.  If there are clients connected to other ports on the multicast router then these clients will see the media stream from the router.

Connecting this way means that you are using the router only to participate in the IGMP protocol to send out queries and reports since the L2 switch cannot initiate these messages and the L2 switch is used to snoop on those packets to handle the membership and packet forwarding only.  The router can also be used to repeat the media stream to other network.

This setup can work for you if your server is setup to not send out the stream until there is at least one client who has joined the multicast group.  Meaning you join the client first then you initiate the stream.

If you intend to send the stream out all the time and let the clients connect when they want (not sure why you do this since you will use up resources of the switch and routers and server even when there is no one to consume to the data) then I think you might try the following setup:

• Connect the router to the L2 switch. 
• Connect your clients to the L2 switch. 
• Connect the media server to the multicast router.

Now the router will see the stream and will not send out the stream until it hears a JOIN from some client.  When it hears a join it will send out the multicast stream to the L2 switch, since the L2 switch has also heard the JOIN it will only forward to the clients.  In this case everything will work the way you want it, meaning that the router will not send the stream to the L2 switch until at least one client has joined.

Anyway, if you need additional help, please send a network diagram along with detail requirements for your application.  Also please let us know the make/model of the router you are using and the media stream server.  Also let us know your network setup (IP address, VLAN, etc).

+--------+     +--------+
| Host A |     | Host C |
+--------+     +--------+
      |            |
  +------+     +------+
  | 3348 |-----| 6024 |
  +------+     +------+
      |            |
+--------+     +--------+
| Host B |     | Host D |
+--------+     +--------+

Message Edited by jrizzo on 07-22-2005 05:23 PM

Please review the rest of the thread.  I have posted a link to a document as well as detail explanation on how an L2 and L3 switches support multicasting.  Please review the information provided.  I believe the thread answers your questions.  The key is that an L2 switch will send multicast to all ports unless it sees a JOIN so if you connect the multicast stream directly to an L2 switch you must make sure your client join before you start the multicast stream on your server (assuming you configured everything correctly - again please see the document I referenced earlier).

Cuong.