Quick way to block mulitcast on a 6509 switchport?

Unanswered Question
Feb 29th, 2008


Have a 6509, sup720. I have a vlan with a multicast stream flooding it. Cannot use IGMP as a few of the switchports are connected to old devices which do not do IGMP, but need to listen to the MC stream. I have a few ports in that same vlan which I really need the multicast stream to not leave. (and I cannot move these devices)

Any easy way to filter out multicast from hitting a port on a vlan saturated with MC traffic?


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gilbert.jason Fri, 02/29/2008 - 21:19


Thank you. I read through it. It seems this solution is for IGMP based multicast. My clients are not using IGMP, they are simply passively listening to a stream broadcasted into the vlan.

While searching around I found: switchport block multicast

I'm going to try this tomorrow at work. Think this will work??



gilbert.jason Fri, 02/29/2008 - 21:42


These are old devices with ports on the same vlan as the MC transmitter. By spraying MC into the vlan all ports receive it(acts like broadcast). It's a horrible implementation, but these clients are really old and do not support IGMP.

We have some other devices(on the same vlan) whom are also getting the MC traffic and are suffering, and we would like to filter it out.

Thank you for your help!


Edison Ortiz Fri, 02/29/2008 - 21:48

You are only able to filter IGMP groups on Layer2 interfaces.

There is another option to filter multicast traffic (read multicast boundary) but only works on routed (layer3) interfaces.

I don't understand your application but in order to receive multicast traffic, one must join the group, else it isn't multicast but broadcast traffic.



gilbert.jason Sat, 03/01/2008 - 05:18

thanks Edison,

The multicast stream is data shot into the vlan by one device. It floods the vlan, and the non IGMP devices can hear it. No need for IGMP. This stuff is VERY old pre-IGMP. We are exploiting multicast's ability to act as broadcast in this subnet. I just need a way to kill the MC traffic on a couple of the ports on the same vlan...

I'll check into the MC boundary stuff. Thank you!


andrew.burns Mon, 03/03/2008 - 04:40


I don't think that the command "switchport block multicast" is supported on the 6500 platform, but "switchport block unicast" will probably have the same effect.

Try it out and let us know how you get on.




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