HP 6120XG HP ProCurve Series 6120 Blade Switches Multicast and Routing Guide - Page 41
Example of Automatic Fast-Leave IGMP Criteria
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Multimedia Traffic Control with IP Multicast (IGMP) How IGMP Operates In the next figure, automatic Fast-Leave operates on the switch ports for IGMP clients "3A" and "5A", but not on the switch port for IGMP clients "7A" and 7B, Server "7C", and printer "7D". Fast-Leave IGMP automatically operates on the ports connected to IGMP clients 3A and 5A, but does not operate on the port connected to Switch 7X because the switch detects multiple end nodes on that port. Fast-Leave IGMP activates on these two ports. 3A 5A Routing Switch Acting as Querier A1 A3 A4 A6 Switch 7A 7B Server 7C Switch 7X Printer 7D Fast-Leave IGMP does not activate on this port. Figure 2-3. Example of Automatic Fast-Leave IGMP Criteria When client "3A" running IGMP is ready to leave the multicast group, it transmits a Leave Group message. Because the switch knows that there is only one end node on port A3, it removes the client from its IGMP table and halts multicast traffic (for that group) to port A3. If the switch is not the Querier, it does not wait for the actual Querier to verify that there are no other group members on port A3. If the switch itself is the Querier, it does not query port A3 for the presence of other group members. Note that Fast-Leave operation does not distinguish between end nodes on the same port that belong to different VLANs. Thus, for example, even if all of the devices on port A6 in figure 2-3 belong to different VLANs, Fast-Leave does not operate on port A6. Default (Enabled) IGMP Operation Solves the "Delayed Leave" Problem. Fast-leave IGMP is enabled by default. When Fast-leave is disabled and multiple IGMP clients are connected to the same port on an IGMP device (switch or router), if only one IGMP client joins a given multicast group, then later sends a Leave Group message and ceases to belong to that group, the switch automatically retains that IGMP client in its IGMP table and continues forwarding IGMP traffic to the IGMP client until the Querier triggers confir mation that no other group members exist on the same port. This delayed leave operation means that the switch continues to transmit unnecessary multicast traffic through the port until the Querier renews multicast group status. 2-15