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New Member

Multicast in a pure layer 2 enviroment

I'm trying to design a switch network for a bunch of digital video cameras and viewing workstations that use multicast.

The idea is that camera's transmit a multicast signal onto the ethernet network and the workstation video software connects to a specific camera by (i assume) joining a multicast group.

I know a requirement of the software is that the switches must support IGMP. All cisco switches support IGMP snooping however all the configuration inforamtion i can see specifically relates to having to configure a multicast router.

My questions is, is a router required for multicasting when you only have a layer 2 network ?

The design in questions puts all hosts on the same subnet and vlan however spread them over multiple switches. In this case what actually keeps track of which switch ports are connected to a certain multicast group and what stops these multicast packets being broadcast out every switch port ?

My other question is how does multicast or igmp for that matter operate in a pure switched network ?


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New Member

Re: Multicast in a pure layer 2 enviroment


No, you don't have to use a router.

Just enable IGMP snooping on your switch.

For cisco run

conf t

ip igmp snooping


sh ip igmp groups

command will show all existing groups and host connected to theese groups

Good luck

Re: Multicast in a pure layer 2 enviroment

Without a mutlicast router in the mix then you will just get flooding of the multicast traffic to all ports in the VLAN. It will still work however but if you put a sniffer on an access port in the VLAN you will see the multicast traffic. Without an IGMP router your IGMP snooping switches won't know what to do so will just flood the multicast traffic.

Have you not got at least 1 Layer-3 switch or router that you can enable multicast on?



New Member

Re: Multicast in a pure layer 2 enviroment

I'm having a similar issue with multicasting. I have a number of Cisco 2950 switches in one of my schools and the school is connected to the rest of the district with a Cisco 2600 router. We just installed a camera system that requires multicast to function. I'd like to configure multicast routing to cut down on the traffic but I'm not sure how to do this on the router. At some point in the future I?ll also need the multicast traffic to cross the router to other buildings in the district.

Cisco Employee

Re: Multicast in a pure layer 2 enviroment

On the 2600, you just need to enable "ip multicast-routing" in global configuration mode and "ip pim sparse-mode" on the interfaces where the multicast source and receiver are.

Hope this helps,

New Member

Re: Multicast in a pure layer 2 enviroment

No not yet.

The network is specifically designed for Video at the same site so there are no vlans, subnets or routing.

It just doesn't seem to make sense to me. If the host can send an igmp join, why can't the switch just use igmp snooping to determine which switch ports it should send the mutlicast packets to. I understand that if your routing multicast traffic then the router should need to know which ports to send multicast packets as by default it drops broadcasts.

I guess I might have to look at making sure one of the switches can act as a Multicast router. Any idea of which model switch will support acting as a multicast router ? Can use a 3560 or would i need a 3750 or higher ?


Re: Multicast in a pure layer 2 enviroment

A catalyst 3560 or 3750 will work but you must be running the IP Services or Advanced IP Services image (EMI). Inter-VLAN multicast routing isn't available in the IP Base (SMI) image.

For IGMP snooping to work your IGMP Snooping switches must know where the multicast routers are:



New Member

Re: Multicast in a pure layer 2 enviroment

Thanks for the help but I found some further information. Some switches (2955, 2960, 3560 or 3750) can act as a "multicast querier", taking place of a multicast router without actually needing to support multicast routing. This means you dont actually need IP Services IOS or a layer 3 switch.

Configuation as per below;

Supported platforms below (look for igmp querier);

Thanks for putting me on the right track however.