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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 17 Co nfiguring IGMP S noo ping and M VR
Understanding M ultic ast VLA N Regi stration
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
elim inates the rece iver port as a forwardi ng desti nati on for this gr oup.
If the Immediate-Leave feature is enabled on a receiver port, the port leaves a multicast group more
quickly. With out Im m ed iate Lea ve, when th e switch r ec eives an I GMP leav e message fr om a s ubscr ib er
on a receiver port, it sends out an IGM P query on tha t port and waits for IG MP group membership
report s. If no repo rt s ar e re ceived in a c onfigu red t im e pe riod, th e re ceiver port is re m oved from
multic ast g rou p me mb ershi p. With Imme dia te L eave, an IG MP query i s not se nt f rom th e re ceiver port
on which the IGMP leave was received. As soon as the leave message is received, the receiver port is
remo ved fro m multicast gr oup membership, wh ich speeds up lea ve laten cy. Enable the Immediate Le av e
featu re only on re ceiver ports to whic h a single receiver device is connect ed.
Figure 17-3 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 2950
switch Catalyst 2950
switch
Catalyst 3550 switch SP2
RP1 RP2 RP3 RP4 RP5 RP6 RP7
74593
S1