tmnxSrrpOperMasterDownInterval 1.3.6.1.4.1.6527.3.1.2.40.1.9.1.18

The value of tmnxSrrpOperMasterDownInterval indicates the time interval for the backup router to declare the master as down. It is calculated as (3 * Advertisement_Interval) + Skew_time where Skew_Time is calculated as (256 - Priority) / 256.