Multicast at Layer2 - when to enable pim on SVI

I’m missing a link when it comes to multicast at layer 2.
The goal is to connect a receiver to a server. The receiver is connected to a port on a layer 2 switch using VLAN 100 and the switch connects to another switch using a DOT1Q trunk. The upstream trunk connects to another Core switch using EIGRP.
The Core switch then connects to another site Via EIGRP and then to a switch using DOT1Q and then to the MCAST Server on vlan 110.

Connection overveiw.
RCVR-> vlan100- SWA-> dot1q- SWB-> EIGRP- CoreSW->EIGRP (MAN L2) - HUBCoreSW->dot1q-SW1->vlan110- MCAST Server
NOTE: CoreSW is the RP & PIM sparse-mode is used

It is my understanding that IGMP works on Layer 2 to get the MCAST traffic to the Layer 3 device.
in this case SWA to SWB IGMP is used. from SWB to CoreSW & HUBCoreSW PIM is used.
The SWB RP then sends MCAST traffic to HUBCoreSW and the HUBCoreSW sends to SW1
Do i need to turn on IP PIM sparse-mode for

The part i’m missing:

  1. Am i required to enable sparse mode on the VLAN 100 ON SWB where the SVI is located
  2. Am i required to enable spare mode on VLAN 100 on SW1 where the SVI is located
  3. The layer 2 switches should be ok the way they are with IGMP version 2 enabled

I have read the layer 2 lessons and will read again. I’m just so unclear on when to enable sparse-mode on the SVI or if there is something i’m missing.

Hopefully the example makes sense and you are able to understand my question.
Clear as Mud as it may be. lol.

Just looking for light to be shed. Thanks in advance for your help.

Hello James

Each L2 subnet within which multicast traffic will be transmitted must be approached independently. If no configurations for L2 multicast are applied, any multicast traffic destined for a host within that L2 domain will be treated as broadcast traffic. The way in which this is achived is using DHCP snooping as shown in the following lesson.

Now in the lesson, you’ll see the first scenario shows how to configure it with a router, and this is your scenario. The router in your case is an L3 switch (SWB if I’m not mistaken). So to answer your first question, when you have an L2 domain that will be serving multicast traffic, and you have a multicast router (which also acts as the default route for unicast traffic), then yes, you must enable PIM on the interface serving that L2 domain.

I believe this answers your second question as well, which is “no”. PIM is only enabled on the interface acting as the default gateway for your L2 domain, which will also be performing multicast routing. The VLAN 100 SVI on SW1 is not performing routing, at least as far as I understand from your description.

And the answer to your third question is yes. You can see the above-linked lesson for more info on details concerning that configuration.

I hope this has been helpful!


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Thanks for your reply. I am reviewing the information provided

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