In certain applications where Multicast Ethernet traffic is prevalent with the network and VLANs are also deployed, just relying on the Ethernet switches to deliver the Multicast traffic to all subscribers is almost impossible without some kind of mechanism for Multicast traffic to span multiple VLANs on demand.
Ethernet switches treat Multicast traffic in a similar way to Broadcast traffic, and the addition of VLANs determines the scope of any multicast traffic. In order to ensure multicast traffic is delivered to ports on different switches that might be on different VLANs there are a number of solutions. One of those solutions is to use a combination of IGMP (Internet Group Management Protocol) and MVR (Multicast VLAN Registration).
Common applications for the use of MVR are IPTV, and Media on Demand networks where a Multicast VLAN is created and the traffic can be shared amongst subscribers regardless of the VLAN to which they belong. Effectively, devices in multiple VLANs can subscribe to receive traffic from a multicast VLAN. This ensures that traffic from the multicast VLAN is isolated from subscriber VLANs and only delivered to the subscriber VLANs on demand.
The subcriber ports on the Ethernet switches use the IGMP mechanism of Join and Leave messages to subscribe and unsubscribe from the multicast streams. The Ethernet switches are enabled for IGMP snooping whereby the switches build forwarding tables based on the contents of the IGMP messages, with MVR reacting to the Join and Leave messages from multiple multicast groups. The use of IGMP and MVR allows selected multicast traffic to cross VLAN boundaries.
MVR operates with hosts on any number of different VLANs in a Layer 2 network. The multicast traffic such as IPTV can be selectively delivered to hosts regardless of the VLAN they belong to. If a particular VLAN has no current hosts requesting the multicast stream then it will not be delivered to that VLAN. Hosts must select the particular multicast stream by sending an IGMP join message, and then an IGMP leave message to indicate termination of the stream.
To configure MVR on a switch, a Multicast VLAN is configured by assigning a number of multicast group addresses to the Multicast VLAN. The VLANs that host the potential receivers are configured as receiver VLANs, which receive the multicast streams on demand from the MVLAN via receiver reports.
On most Cisco catalyst switches, two modes of MVR are available, Dynamic and Compatible:
In dynamic mode, the switch is set to perform IGMP snooping with the IGMP join and leave packets being sent to the switch CPU. The join messages are sent to the multicast enabled router so that the router can determine which multicast data streams need to be forwarded out the interface to the switch. Receiver ports become members of their respective multicast VLAN groups for the MVR control and data traffic. IGMP reports are forwarded out the source ports for specific VLANs.
In compatible mode, the switches operate in a similar way to dynamic mode, except that IGMP report packets for MVR groups are not sent out of the multicast VLAN source ports, and the router must be statically configured in order that the interface can receive the multicast streams.
MVR is discussed in a number of our instructor-led training courses.