11-8
Catalyst 2950 Desktop Switch Software Configuration Guide
78-11380-03
Chapter 11 Configuring IGMP Snooping and MVR
Understanding Multicast VLAN Registration
The Catalyst 2950 switch has dynamic and compatible modes of MVR operation:
• 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 only forwards multicast streams for a particular group to an
interface 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 interoperates with Catalyst 2900 XL and Catalyst 3500 XL
switches. It works the same as dynamic 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 router. 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.
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 connected to one subscriber port, which is a switch port
configured as an MVR receiver port. Refer to Figure 11-3. 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 configured multicast
MAC addresses, the switch CPU modifies the hardware address table to include this receiver port and
VLAN as a forwarding destination of the specified multicast stream when it is received from the
multicast VLAN. Uplink ports that send and receive multicast data to and from the multicast VLAN are
called MVR source ports.
When a subscriber changes channels or turns off the television, the set-top box sends an IGMP leave
message for the multicast stream. The switch CPU sends an IGMP group-specific query through the
receiver port VLAN. If there is another set-top box in the VLAN still subscribing to this group, that
set-top box must respond within the maximum response time. If the CPU does not receive a response, it
eliminates the receiver port as a forwarding destination for this group.
If the Immediate-Leave feature is enabled on a receiver port, the port leaves a multicast group more
quickly. Without Immediate Leave, when the switch receives an IGMP leave message from a subscriber
on a receiver port, it sends out an IGMP query on that port and waits for IGMP group membership
reports. If no reports are received in a configured time period, the receiver port is removed from multicast
group membership. With Immediate Leave, an IGMP query is not sent from the receiver port on which
the IGMP leave was received. As soon as the leave message is received, the receiver port is removed
from multicast group membership, which speeds up leave latency. Only enable the Immediate Leave
feature on receiver ports to which a single receiver device is connected.
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