Symptom

One of two symptoms may be seen as a result of this issue: 1. A line of interfaces used in 'monitor session # source interface' configuration will go missing following a reload (typically seen with reloads for software upgrade) 2. ISSU may fail due to an "MCL" Failure - Where "show redundancy config-sync failures historic mcl" shows a mismatch between monitor session configs.

Conditions

This can be seen on any Catalyst 9000 series switch when performing an update, and is not specific to certain versions or software trains. This is also specific to having a monitor session configuration that is large enough to cause the config to wrap to a second line. Typically, this is most common when defining a large number of source interfaces. Example: monitor session 10 source interface monitor session 10 source << This configuration may go missing. The 2nd (or any additional) lines must be present to trigger this issue. This defect is not applicable to configurations where all possible source interfaces fit into a single line.

Workaround

Prior to a software upgrade remove or adjust the configuration so that it fits on a single line to avoid failure. Otherwise if after a normal upgrade the configuration is missing, use the startup-configuration as a reference and re-paste the configurations to the switch. You may need to specify "source interface " and not just "source to have multiple lines be appended.

Further Problem Description