This indicates which interface was affected by a logged FRR event, and it may be set to 0 if mplsFrrConstProtectionMethod is set to oneToOneBackup. It ties a logged event back to a specific ifIndex whose failure triggered it, which is the fastest way to identify a chronically flapping link when scanning the log table, though on a one-to-one-backup router this field will not help localize the cause since it is defined to read 0 there. For example, a log entry showing cmplsFrrLogInterface equal to 14 immediately points the admin at ifIndex 14's cable or its neighboring node as the source of that failover, without needing to correlate against separate interface counters.