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Multicast fails when receiver transmits same multicast stream it joined via IGMP



Article ID: KB16047 KB Last Updated: 12 Feb 2010Version: 1.0
Multicast fails when receiver transmits same multicast stream it joined via IGMP
Customer has a unique application that requires two-way multicast communication between different application servers.

Server A ---- PIM router-----RP-------- (untrust) Firewall (trust) -----Server B

Server A is the sender and is transmitting stream A
Server B is the receiver and it sends an IGMPv2 join for stream A
Server B is also transmitting stream A for another receiver on the untrust side of the firewall.
Firewall is running PIM on the untrust as well as the trust interfaces and IGMPv2 on the trust interface.

The multicast configuration is correct but the Server B never receives the multicast stream A from Server A through the firewall.
In the above network design the multicast RP or source tree is not built because the firewall does not send the PIM join towards the RP.

To ensure that the multicast traffic works with the help of dynamic PIM mroutes, the application servers should NOT transmit mcast streams for which they send out IGMP joins.

If that happens, the firewall will NOT send the PIM join toward the RP as it receives the IGMP join as well as the native multicast stream on the same interface therefore the Multicast tree is NOT built and Server B fails to receive mcast stream A from Server A.

The work-around for that is to ensure that the server B sends IGMP joins and mcast stream A on different vlans.

An alternate work-around is to add static mroutes for all devices between the source and receiver but that could result to heavy administrative load.
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