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ROS™ v3.5 198 RS400
If RSTP detects change in the network topology, IGMP will take some actions to avoid loss of
multicast connectivity and reduce network convergence time:
• The switch will immediately issue IGMP queries (if in IGMP Active mode) to obtain potential
new group membership information.
• The switch can be configured to flood multicast streams temporarily out of all ports that are
not configured as RSTP Edge Ports.
8.1.3 Combined Router and Switch IGMP Operation
This section describes the additional challenges of multiple routers, VLAN support and
switching.
Producer P1 resides upon VLAN 2 while P2 resides upon VLAN 3. Consumer C1 resides upon
both VLANs whereas C2 and C3 reside upon VLANs 3 and 2, respectively. Router 2 resides
upon VLAN 2, presumably to forward multicast traffic to a remote network or act as a source of
multicast traffic itself.
Multicast
Router 1
P1
C1 C2 C3
Switch
Multicast
Router 2
VLAN 2,3 VLAN 3 VLAN 2
VLAN 2
VLAN 2
P2
VLAN 3
VLAN 2
Figure 136: IGMP Operation Example 2
In this example we will assume that all the devices agree that router 1 is the querier for VLAN 2
and router 2 is simply a non-querier. In this case, the switch will periodically receive queries
from router 1 and, thus, maintain the information which port links to the multicast router.
However, the switch port that links to router 2 must be manually configured as “router port”,
otherwise, the switch will not send neither multicast streams or joins/leaves to router 2.
Note that VLAN 3 does not have an external multicast router. The switch should be configured
to operate in its “routerless” mode and issue general membership queries as if it is the router.
Processing Joins
If host C1 desires to subscribe to the multicast streams for both P1 and P2, it will generate two
joins. The join from C1 on VLAN 2 will cause the switch to immediately initiate its own join to
multicast router 1 (and to issue its own join as a response to queries).
The join from C1 for VLAN 3 will cause the switch to immediately begin forwarding multicast
traffic from P2 to C2.