Solved: Layer 2 Multicast configuration?

dan hale · ‎03-12-2016

Hi All, from my past experience and everything that I have read there is no additional configuration for multicast to work when the receivers and the device that is doing PIM are in the same subnet? Is this correct?

I have a PBX and voice subnet. The device that is running PIM is in the same subnet that my receivers (phones) are in. However, my phone guy is telling me that I need to enable multicast to do additional functions for the phones.

I have enabled multicast in the past on some of my layer 3 devices but, only when the device that is trying to send multicast packets are moving between subnets such as imaging software.

Is there any additional configuration I need on my switches, they are all Cisco 3560 LAN base switches.

Is there any troubleshooting tips that I can do on the switches to show multicast packets coming or not coming into the receiver's?

Thanks,

Dan

Peter Paluch · ‎03-12-2016

Hi Dan,

Cisco Catalyst switches indeed do not require additional configuration to pass multicast inside a VLAN, and by definition, no additional mechanism is needed for a multicast to be flooded across a VLAN.

However, Catalyst switches do run IGMP Snooping by default, and it is possible (though not certain) than this could be causing trouble. IGMP Snooping tries to optimize multicast flooding by learning about attached receivers and multicast groups they have subscribed to, and then forwarding the multicast only through those switchports that have receivers for the corresponding group connected to them. IGMP Snooping depends on the presence of a multicast-enabled router in a VLAN to send periodic IGMP Membership Query messages. If such router does not exist, IGMP Snooping should lie dormant but the experience has been uneven at times.

So you could try to deactivate the IGMP Snooping on your switches just in case by simply entering the no ip igmp snooping in the global configuration mode. This won't cause any outage in your normal network operation.

If this does not help then please provide us with more information about the multicast flow (source, destination group) as well as the exact symptoms you are experiencing.

Best regards,
Peter

View solution in original post

Peter Paluch · ‎03-12-2016

Hi Dan,

Cisco Catalyst switches indeed do not require additional configuration to pass multicast inside a VLAN, and by definition, no additional mechanism is needed for a multicast to be flooded across a VLAN.

However, Catalyst switches do run IGMP Snooping by default, and it is possible (though not certain) than this could be causing trouble. IGMP Snooping tries to optimize multicast flooding by learning about attached receivers and multicast groups they have subscribed to, and then forwarding the multicast only through those switchports that have receivers for the corresponding group connected to them. IGMP Snooping depends on the presence of a multicast-enabled router in a VLAN to send periodic IGMP Membership Query messages. If such router does not exist, IGMP Snooping should lie dormant but the experience has been uneven at times.

So you could try to deactivate the IGMP Snooping on your switches just in case by simply entering the no ip igmp snooping in the global configuration mode. This won't cause any outage in your normal network operation.

If this does not help then please provide us with more information about the multicast flow (source, destination group) as well as the exact symptoms you are experiencing.

Best regards,
Peter