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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter11 Configuring IGMP Snooping and M VR 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 IGM P gr oup -sp ecifi c qu er y th rou gh 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 th e sw itch re ceive s an IGM P leave messa ge from a subsc ribe r
on a receiver port, it sends out an IGMP query on tha t p ort an d w ai ts fo r IGM P gr oup m emb ershi p
reports. If no reports are received in a configured time period, the receiver port is removed from
multicast group membership. With Immediate Leave, an IGM P 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 th e Immediate Leav e
feature only on receiver ports to which a single receiver device is connected.
Figure11-3 Multicast VLAN Registration Example
SP1
Multicast
data Multicast
data
Customer
premises
Multicast VLAN
SP
SP
RP = Receiver Port
SP = Source Port Note: All source ports belong to
the multicast VLAN.
Hub
TV
data
Set-top boxSet-top box
TV TV
PC
SP
SP
SP
SP
IGMP join
Cisco router
Multicast
server
Catalyst 3550 switch
Catalyst
2900/3500 XL
switch
Catalyst 3550 switch
Catalyst
2900/3500 XL
switch
SP2
RP1 RP2 RP3 RP4 RP5 RP6 RP7
53032
S1