Cisco – IGMP snooping across VLANs

ciscoigmpmulticastswitchingvlan

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)?

Best Answer

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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