Chapter 5
Setup and Configuration
RUGGEDCOM ROS
User Guide
220 IGMP Concepts
If RSTP detects a change in the network topology, IGMP will take some actions to avoid the 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.
Combined Router and Switch IGMP Operation
The following example illustrates the challenges faced with multiple routers, VLAN support and switching.
Producer P1 resides on VLAN 2 while P2 resides on VLAN 3. Consumer C1 resides on both VLANs whereas C2 and
C3 reside on VLANs 3 and 2, respectively. Router 2 resides on VLAN 2, presumably to forward multicast traffic to a
remote network or act as a source of multicast traffic itself.
Figure151:Example – Combined Router and Switch IGMP In Operation
1.Producer 2.Multicast Router 1 3.Multicast Router 2 4.Switch 5.Host
In this example:
• P1, Router 1, Router 2 and C3 are on VLAN 2
• P2 and C2 are on VLAN 3
• C1 is on both VLAN 2 and 3
Assuming that router 1 is the querier for VLAN 2 and router 2 is simply a non-querier, the switch will periodically
receive queries from router 1 and maintain the information concerning which port links to the multicast router.
However, the switch port that links to router 2 must be manually configured as a router port. Otherwise, the
switch will send neither multicast streams nor 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 wants to subscribe to the multicast streams for both P1 and P2, it will generate two membership
reports. The membership report from C1 on VLAN 2 will cause the switch to immediately initiate its own
membership report to multicast router 1 (and to issue its own membership report as a response to queries).
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