Cisco Catalyst 2950 Software Configuration Manual page 376

Understanding Multicast VLAN Registration
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.
Figure 16-3 Multicast VLAN Registration Example
Catalyst
2950 switch
SP
Catalyst 2950 Desktop Switch Software Configuration Guide
16-12
Catalyst
2950 switch
SP
SP
SP1
Multicast
data
RP1 RP2 RP3 RP4 RP5 RP6 RP7
Customer
premises
IGMP join
Set-top box
TV
data
TV
RP = Receiver Port
SP = Source Port
Cisco router
Multicast
server
SP
SP
Catalyst
2950 switch
SP2
S1
Hub
Set-top box
PC
TV
Note: All source ports belong to
the multicast VLAN.
Chapter 16 Configuring IGMP Snooping and MVR
Catalyst
2950 switch
SP
Multicast
data
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