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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter21 Configuring IGMP Snooping and MVR
Understanding Multicast VLAN Registr a t ion
Understanding Multicast VLAN Registration
Multicast VLAN Registration (MVR) is designed for applicati ons usi ng wi de-sc ale dep loymen t of
multicast traffic across an Ethernet ring-based service provider network (for example, the broadcast of
multiple television channels over a service-provider network). MVR allows a subscriber on a port to
subscribe and unsubscribe to a multicast stream on the network-wide multicast VLAN. It allows the
single multicast VLAN to be shared in the network while subscribers remain in separate VLANs. MVR
provides the ability to continuously send multicast streams in the multicast VLAN, but to isolate the
streams from the subscriber VLANs for bandwidth and security reasons.
MVR assumes that subscriber ports subscribe and unsubscribe (j oin an d le ave) these mu lti cast stream s
by sending out IGMP join and leave messages. These messages can originate from an IGMP
version-2-compatible host with an Ethernet connection. Although MVR operates on the underl ying
mechanism of IGMP snooping, the two features operate independently of each other. One can be enabled
or disabled without affecting the behavior of the other feature. However, if IGMP snooping and MVR
are both enabled, MVR reacts only to join and leave messages from multicast groups configured under
MVR. Join and leave messages from all other multicast groups are managed by IGM P snoopi ng.
The switch CPU identifies the MVR IP multicast streams and their associated MAC addresses in the
switch forwarding table, intercepts the IGMP messages, and modifies the forwarding table to include or
remove the subscriber as a receiver of the multicast stream, even though the re ceivers migh t be in a
different VLAN from the source. This forwarding behavior selectively allows t raffic to cro ss b etw een
different VLANs.
The switch has these modes of MVR operation: dynamic and compatible.
•When operating in MVR dynamic mode, the switch performs standard IGMP snooping. IGMP
information packets are sent to the switch CPU, but multicast data packets are not sent to the CPU.
Dynamic mode allows the multicast router to run normally because the switch sends the IGMP join
messages to the router, and the router forwards multicast streams for a particular group to an
interface only if it has received a join message from the interface for the group. Receiver ports are
treated as members of the multicast VLAN for MVR multicast control and data traffic. IGMP reports
for MVR groups are sent out source ports in the multicast VLAN.
•When in MVR compatible mode, MVR on the Catalyst 2950 or Catalyst 2955 switch interoperates
with MVR on Catalyst 3500 XL and Catalyst 2900 XL switches. It works the same as dynam ic mode
for all multicast data packets and IGMP query and leave packets. However, received IGMP report
packets for MVR groups are not sent out on the multicast VLAN source ports. In contrast to dynamic
mode, the switch does not send join messages to the route r. The router must be statically configured
for the interface to receive the multicast stream. Therefore, in this mode, MVR does not support
dynamic membership joins on source ports.
Note IGMPv3 join and leave messages are not supported on switches running MVR.