Symptom: Noticeable slowdown in some multicast traffic

If the switch is supporting more than 1022 active flows, this generates the message Unable to learn HW IP multicast groups, table FULL in the Event Log, because there is no room in the hardware MRT to add another multicast group. Software will route any multicast packets sent to multicast groups that are not in the hardware MRT, but it will be slower, and packets may be dropped if the data rate is greater than 3000 packets per second. See Flow capacity.


The PIM protocol uses oneMRT entry for every IP multicast S/G pair that it is routing. An entry is not used if the multicast flow is bridged and not routed. Entries in this table are automatically aged-out if they are unused for a period of time.