Multicast - PIM join message during register stop suppression timer

In a multicast topology where PIM sparse mode is employed, if a host wishes to recieve multicast traffic from a particular multicast group, it will send a join message to its local router. This will be forwarded to the RP. If a PIM Register-Stop message had previously been sent to the first-hop router (the router connected to the multicast source), the RP will typically wait for the first-hop router to send a new PIM Register message. This is assuming a strict adherence to the “rules” of this operation.

This means that the host wanting to join will have to wait for the suppression timer to elapse so that the first-hop router will start sending multicast traffic to the RP once again.

However, this seems inefficient. This was put to the test in a lab, and it was found that if a join message reaches the RP during that suppression timer, the RP will automatically send a join message to the first hop router, informing it that it wants to receive traffic. The first hop router in turn will ignore its timer and will begin sending multicast traffic.

This is highly vendor and platform specific, and it may be that some IOS versions or other vendors don’t do this while others do. It is likely that most modern implementations do this to avoid the inefficiencies involved.