Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
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The appropriate border routers must be used for each VLAN, or PIM-DM
will not forward the traffic. This could occur when multiple border routers
exist. It may be necessary to configure multiple overlapping domains if
the multicast source address can generate the same multicast address and
have different best paths to the PIM-DM domain.
C a u t i o n
Be careful to avoid configuring a IGMP forward loop, as this would leave the
VLANs in a joined state forever once an initial join is sent from a host. For
example, a join is issued from the host in VLAN 2 and routing switch 2 will
proxy the join onto VLAN 1. Routing switch 3 will then proxy the join back
onto VLAN 2 and increment its internal count of the number of joins on VLAN
2. Even after the host on VLAN 2 issues a leave, the proxy join will continue
to remain and refresh itself each time a query occurs on VLAN 2. This type of
loop could be created with multiple routers if an IGMP proxy is allowed to get
back to the VLAN of the router that initially received the IGMP join from a
host. (See figure 2-10.)
Routing Switch 2
Routing Switch 3
Routing Switch 1
(Border router)
PIM SM DOMAIN
PIM DM DOMAIN
VLAN 1
VLAN 2
Proxy VLAN 2 to 1
Proxy VLAN 1 to 2
Figure 2-10. Proxy Loop Scenario
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