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The "%MLSM-6-MMLS_MODE_CHANGE: MMLS:" error message appears in Catalyst 6500 series switches running Cisco IOS system

Core issue

The complete error message looks like this:

%MLSM-6-MMLS_MODE_CHANGE: MMLS: Replication Mode Change Detected because oir event

This means that the multicast replication mode of the switch has changed.  There are two ways to replicate traffic in the 6 k:

  • Ingress Ingress replication ASIC creates all needed copies of the multicast packet, and all are sent to their respective ports.

  • Egress Ingress replication ASIC only sends one copy of each packet to each egress linecard, and the replication ASIC of the egress linecard creates additional copies for all ports that need the stream.

Only certain linecards support egress replication, so only egress replication mode is chosen, if all linecards in the system support this mode, or else its in ingress.  The syslog you see is the syslog that prints as soon as you insert a linecard in the system that does not support egress replication, as the mode needs to change to ingress.

In some cases, this error message is displayed after a loopback interface is created.

Resolution

When you configure loopback interfaces for multicast, the system no longer supports egress replication, as you cannot predict what the CPU does with the stream.

This is an expected behaviour. This should not disrupt the stability of the system at all; it just makes the switch slightly less efficient at handling multicast.

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