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Set up:

  • Core switch: C1

  • C1 is an L3 switch--there is no router

  • Access switches: A1, A2, A3

  • VLANs: V1, V2

  • Hosts: H11, H12, H13, H21, H22, etc.

Connections:

  • C1 connects to A1, A2, A3

  • A1, A2, A3 each support V1 and V2

  • A1 connects to H11, H12, H13

  • A2 connects to H21, H22, etc.

IGMP snooping is running on core switch and access switches.

The way access switches using IGMP snooping decide to forward multicast data to hosts is relatively obvious.

But how does IGMP snooping look from the Core's perspective? Is there a separate querier per VLAN? Are there essentially two tables (one per VLAN), and each table has zero to three rows (one entry per access switch)?

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Switches with multiple VLANs look like multiple switches, and they maintain separate tables for each VLAN. For example, a switch with two VLANs will have two MAC address tables, one for each VLAN.

You do not get IGMP snooping to cross VLANs, and multicast doesn't cross VLANs unless you enable multicast routing.

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  • Thank you! The hosts on V1 and V2 do communicate via multicast. How does this communication work... Does IGMP snooping on the Core maintain a table of which VLANs are interested? Or is the inter-VLAN routing completely separate from IGMP snooping? – user2588666 Dec 22 '17 at 22:55
  • You will need to edit your question to include the configurations of the switches. We cannot guess how this is working without seeing how the switches are configured. – Ron Maupin♦ Dec 22 '17 at 23:00
  • Understood. I don’t have access to that information, so I was hoping the configuration was obvious. Thanks for your explanations. – user2588666 Dec 23 '17 at 1:27
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    As Ron points out, IGMP happens locally within a subnet (read: bound to a single VLAN). Anything that crosses that boundary is inter-VLAN routing, which means you're using something like PIM to both enable routing on the subnet and allow routers up- or down- stream of C1 to send and receive multicast. – rnxrx Dec 23 '17 at 2:16

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