Avaya has released a technical support bulletin detailing an issue that can impact IST stability in a large Multicast network. I know a number of readers have had issues with Multicast support in extremely large networks.
In large campus networks with SMLT topologies where multicast routing protocols (such as PIM) have been provisioned and scaled to large amounts of multicast senders and receivers, it has been observed that high CPU utilization
(sometimes combined with high CPU buffer utilization) leading to IST instability may occur during re-convergence of the multicast routing protocols after failures.
Release 220.127.116.11 has been modified with changes that were originally introduced in release 18.104.22.168. These changes allow IST protocol messages to be processed even under high CPU utilization. This is achieved by checking to see if IST control messages are queued up (but not yet processed) before deciding that the IST session has timed out and needs to be brought down. Each line card recognizes and counts IST control messages when they arrive and before they are sent to the CP, and the IST message processing logic on the CP will check for outstanding IST control messages before deciding the IST needs to be brought down due to inactivity.