This section discusses the various timers associated with ERPS.

Guard Timer

The guard timer is used to prevent Ethernet ring nodes from acting upon outdated R-APS messages, and to prevent the possibility of forming a closed loop. The guard timer is activated whenever an Ethernet ring node receives an indication that a local switching request has cleared (i.e., local clear SF, clear). The guard timer can be configured in 10 ms steps, between 10 ms and two seconds, with a default value of 500 ms. This timer period should be greater than the maximum expected forwarding delay in which an R-APS message traverses the entire ring. The longer the period on the guard timer, the longer an Ethernet ring node is unaware of new or existing relevant requests transmitted from other Ethernet ring nodes, and is unable to react to them.

A guard timer is used in every Ethernet ring node. Once a guard timer is started, it expires by itself. While the guard timer is running, any received R-APS Request/State and Status information is blocked and not forwarded to the Priority Logic. When the guard timer is not running, the R-APS Request/State and Status information is forwarded unchanged.

Hold-off Timer

W hen a new defect, or more severe defect occurs (new SF), this event is not be reported immediately to protection switching if the provisioned hold-off timer is a non-zero value. Instead, the hold-off timer is started. When the hold-off timer expires, the trail that started the timer is checked to see if a defect still exists. If one does exist, that defect is reported to protection switching. The suggested range of the hold-off timer is 0 to 10 seconds in steps of 100 ms with an accuracy of ±5 ms. The default value for a hold-off timer is 0 seconds.

Delay Timers

In revertive mode, the wait-to-restore (WTR) timer is used to prevent frequent operation of the protection switching caused by intermittent signal failure defects. The wait-to-block (WTB) timer is used when clearing Forced Switch and Manual Switch commands. As multiple Forced Switch commands are allowed to coexist in an Ethernet ring, the WTB timer ensures that clearing of a single Forced Switch command does not trigger the re-blocking of the RPL. When clearing a Manual Switch command, the WTB timer prevents the formation of a closed loop due to a possible timing anomaly where the RPL owner node receives an outdated remote MS request during the recovery process.