Multicast on 2950

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Jan 22nd, 2009
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How I can active multicast on Cisco 2950 because I see nothing in the table

sh mac-addresse table multicats?

I try commands like ip igmp snooping but no results.

Thanks a lot

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Yudong Wu Thu, 01/22/2009 - 13:53
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Do you have multicast source and receiver to connect to the switch?

IGMP Snooping should add entry when it hear "igmp report/join" from receiver or certain pim/igmp message from router.

claude1968 Fri, 01/23/2009 - 04:33
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I have two Sun server (Firewall checkpoint) and the configuration is Load Balancing Multicast. I enable ip igmp snooping vlan and I see nothing in the mac-addresse table multicast?

I don't know why?

Yudong Wu Fri, 01/23/2009 - 08:02
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Since this multicast traffic is only for local subnet, Sun Servers probably sends the multicast packet by converting the multicast IP to multicast MAC directly instead of sending "igmp join". Switch builds its MAC table based on source mac in the packet. Your multicast MAC will be a destination MAC. Therefore, switch never learns it.

andrew.butterworth Thu, 01/22/2009 - 14:06
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A 2950 by default has IGMP snooping enabled for all VLANs, however it needs to see a mutlicast router(s) to actually start filtering traffic. Use the following command and its subcommands to see whats going on:

show ip igmp snooping



claude1968 Fri, 01/23/2009 - 04:36
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I don't have routers just one swtich 2950 and two Sun server (Check Point Firewall in load balancing multicast). Because I'm in lab for the moment. I see nothing appear in the mac-address table multicast.

I don't know why?

Giuseppe Larosa Sat, 01/24/2009 - 02:28
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Hello Claude,

IGMP snooping works by sniffing the IGMP joins/reports or receivers.

If there is no router with multicast routing enabled that sends every 60 seconds an IGMP general query IGMP snooping cannot listen to IGMP reports.

However, this is an optimization of multicast forwarding the two devices should be able to exchange multicast frames with only drawback that frames are flooded to all other ports that are in the same Vlan.

So is communication working between the two devices ?

If not you may need to disable igmp snooping on the vlan where the two servers are connected to revert to a flooding behaviour.

Hope to help



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