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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 17 Configuring IGMP Snooping and M V R Understanding Multicast VLAN Registration

Understandin g Multicast VL AN Registrati on

Multica st VLA N R egistrat ion (M VR) is desi gned f or ap pli cati ons usi ng wi de-sc ale d ep 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
subscr ibe and unsu bscribe to a multica st stream on th e network -wide mul ticast VLAN. It all ows the
single multi cast VLAN to be shar ed in the ne tw o rk while su bsc riber s remai n in separa te VL ANs. MV R
provides the ability to continuously send multicast streams in the multicast VLAN, but to isolate the
stream s fro m th e subsc rib er VL ANs for b an dwi dth a nd sec urit y re as ons.
MVR as sume s th at su bsc ribe r ports subs cr ibe an d un subs cr ibe (joi n an d l eave) thes e mu ltic ast str eam s
by sending out IGMP join and leave messages. These messages can originate from an IGMP
version-2-com pati ble host with an Ethern et conne ction . Although MVR opera tes on the unde rlying
mechanism of IGMP snooping, the two features operate independently of each other . One can be enabled
or disable d without affe ctin g the behavior of the ot her featu re. However, if IGMP sno oping and MVR
are both enabled, MVR reacts only to join and leave messages from multicast groups configured under
MVR. Join and leave message s f rom a ll ot h er m ultic ast group s a re m ana ged by IGMP 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 s ubscr iber as a re ceiver of the mu lti cast s tream , even though th e receivers mi ght be in a
different VLA N f ro m th e sour ce. T his f orwa rd ing behavior sel ect ively allows tra ffic to cro ss b etwe en
different VLANs.
The switc h has these modes of MVR op erati on: dynam ic and co mpatibl e.
•When oper ating in MVR dyn amic mo de, the sw itch perfo rms standard IGMP snoo ping. IGMP
information packets are sent to the switch CPU, but multicast data packets are not sent to the CPU.
Dynamic mode a llows the multica st rou ter to run norm all y bec aus e 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 traf fic. IGMP reports
for MVR groups are sent out source ports in the multicast VLAN.
•When in MVR compatible mode, MVR on the Catalyst 3550 switch interoperates with MVR on
Catalyst 350 0 XL and Catalys t 2900 XL switc hes. It works the same as dynam ic mode for all
multic ast data pac kets and IGMP query and leave packets. However, rec eived IGMP report packets
for MVR groups are not sent out on the multicast VLAN source ports. In contrast to dynamic mode,
the switch d oes not send join me ssages to the rou ter. The rou ter must be statica lly conf igured for the
interface to receive the multicast stream. Therefore, in this mode, MVR does not support dynamic
member ship joins on sour ce por ts.

Using MVR in a Multicast Television Application

In a multicast television application, a PC or a television with a set-top box can receive the multicast
stream. Multiple set-top boxes or PCs can be co nnected to one subscriber port, which is a switch port
configured as an MVR receiv er port. See Figure 17-3. DHCP assigns an IP address to the set-top box or
the PC. When a subscriber selects a channel, the set-top box or PC sends an IGMP report to the S1 switch
to join the appropriate multicast. If the IGMP report matches one of the config ured multicast MAC
addresses, the switch CPU modifies the hardware address table to include this receiver port and VLAN
as a forward ing destination of the specified multicast stream when it is received from the multicast
VLAN. Uplink ports that send and receive multicast da ta to and from the multicast VLAN are called
MVR sourc e po rts.