ERP timers

ERP provides various timers to ensure stability in the ring while a recovery is in progress or to prevent frequent triggering of the protection switching. All of the timers are operator configurable.

• Guard timer -- All ERNs use a guard timer. The guard timer prevents the possibility of forming a closed loop and prevents ERNs from applying outdated R-APS messages. The guard timer activates when an ERN receives information about a local switching request, such as after a switch fail (SF), manual switch (MS), or forced switch (FS). When this timer expires, the ERN begins to apply actions from the R-APS it receives. This timer cannot be manually stopped.
• Wait-To-Restore (WTR) timer -- The RPL owner uses the WTR timer. The WTR timer applies to the revertive mode to prevent frequent triggering of the protection switching due to port flapping or intermittent signal failure defects. When this timer expires, the RPL owner sends a R-APS (NR, RB) through the ring.
• Wait-To-Block (WTB) timers -- The WTB timer is activated on the RPL owner. The RPL owner uses WTB timers before initiating an RPL block and then reverting to the idle state after operator-initiated commands, such as for FS or MS conditions, are entered. Because multiple FS commands are allowed to co-exist in a ring, the WTB timer ensures that the clearing of a single FS command does not trigger the re-blocking of the RPL. The WTB timer is defined to be 5 seconds longer than the guard timer, which is enough time to allow a reporting ERN to transmit two R-APS messages and allow the ring to identify the latent condition. When a MS command is cleared, the WTB timer prevents the formation of a closed loop caused by the RPL owner node applying an outdated remote MS request during the recovery process.
• Hold-off timer -- Each ERN uses a hold-off timer to delay reporting a port failure. When the timer expires, the ERN checks the port status. If the issue still exists, the failure is reported. If the issue does not exist, nothing is reported.
• Message interval -- This is an operator configurable feature for sending out R-APS messages continuously when events occur.