Multicast Question

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Mar 11th, 2008
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Here is the scenario.

A 6509 at the core. Running layer 3. It uplinks (layer 2) to a switch (owned by the state which i believe is layer 3) and routes out to the external network/internet.

All other switches in this school are layer 2. Multicast DOES NOT WORK. Unless i do the following commands on my 6509

ip multicast routring

int vlan 1

ip pim dense-sparse mode

Then it works like a charm. Without that commmand multicast works nowhere. Not even to the switch the multicast server is plugged into. Can anyone explain why? I understood multicast to be similar to a broadcast when it is look at from layer 2. Any insight would be appreciated.

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Kevin Dorrell Tue, 03/11/2008 - 08:23
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I think you are being caught by igmp snooping. When IGMP snooping is enabled, the switch no longer floods the multicasts to all ports, but only to the ports that have actually requested the multicast stream by generating an IGMP report.

When you have IGMP snooping, there is a special type of port called the router port. This port receives all the IGMPs, because the router must see them. The switch detects these router ports because of the PIM protocol. You can see the router port if you do show ip igmp snooping mrouter. In your case, I suppose it would be the pseudo-port of VLAN 1 ... if PIM is running.

If PIM is not working, I'm not sure the IGMP will be passed to the multicast source. If PIM is working, the source registers with the router, and the route handles the IGMP.

Try no ip igmp snooping. Be warned this will flood all multicasts to all ports, but it could tell you whether it is the IGMP snooping that is at thr root of your problem.

Alternatively, you could try ip igmp snooping static 0100.5e12.3456 interface F0/20, substituting the interface where you have your multicast source and the MAC address by the multicast group address.

Kevin Dorrell


mikegrous Tue, 03/11/2008 - 08:26
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from my reading on cisco that is what i assumed it was, however i wanted someone else's insight. Thank you very much.

Kevin Dorrell Tue, 03/11/2008 - 08:33
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Mike, I edited some more into the message, that might give you a way forward.



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