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Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 20 Configuring IGMP Snooping and MVR
Understanding Multicast VLAN Registration
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.
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When in MVR compatible mode, MVR on the Catalyst 3550 switch interoperates with MVR on
Catalyst 3500 XL and Catalyst 2900 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.
Note
IGMPv3 join and leave messages are not supported on switches running MVR.
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. See
. 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. Enable the Immediate Leave
feature only on receiver ports to which a single receiver device is connected.