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Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MSTP (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 13 of 70
Spanning Tree IGMP Fast Recovery
Using the STP IGMP Fast recovery, the Multicast traffic takes advantage of the connectivity and
convergence time provided by the Spanning Tree Protocols.
In
Figure 5
, all switches run both IGMP snooping and any one of the Spanning Tree Protocols -
STP, RSTP, MSTP. The multicast router floods traffic in the topology for multicast groups that the
Client has been subscribed to. A subscription to a specific multicast group is performed by sending
IGMP reports.
Figure 5: Spanning Tree IGMP Example
The multicast router sends an IGMP query to the clients for their multicast group memberships. IP
hosts reply with IGMP Reports. The traffic flows from the router, through Switch D and Switch A,
to Switch C.
The mrouter port (an mrouter port is a port that has received an IGMP query from an upstream
device) of Switch C that links to Switch B is blocked. If a topology change occurs and the link
between Switch C and Switch A goes down, the blocked port of Switch C turns into forwarding
state. If Spanning Tree IGMP Fast Recovery is configured on Switch C, the switch reacts to the
topology change by sending an IGMP General Query to all its non-mrouter ports. The client(s)
respond to the General Query with an IGMP report. Switch C forwards the IGMP report to its
mrouter ports and the report goes to the multicast router through Switch B and Switch D. As a