This is the GMRP Join timer for a bridge port, expressed in centiseconds, defaulting to 20. An admin tunes it to control how long the port waits before sending a Join message when propagating multicast group registrations, affecting how quickly GMRP state converges across the switch. Increasing it from the default 20 centiseconds to 40 would slow down join propagation on that port, useful on a link where duplicate Join messages need to be suppressed.