The guard timer, configured by means of the guard-time command, prevents ERNs from acting upon outdated R-APS messages and prevents the possibility of forming a closed loop. The guard timer enforces a period during which an ERP topology ignores received R-APS.
This timer period should always be greater than the maximum expected forwarding delay in which an R-APS message traverses the entire ring. The longer the period of the guard timer, the longer an ERN is unaware of new or existing relevant requests transmitted from other ERN and, therefore, unable to react to them.
The guard timer is used in every ERN, 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.
Note
The ITU-T G.8032 standard defines the guard timer period as configurable in 10 ms increments from 10 ms to 2000 ms (2 seconds) with a default value of 500 ms.The guard timer is activated when an ERN receives an indication that a local switching request, such as a clear signal fail, manual switch, or forced switch, is cleared.
The guard timer can be configured in 100-ms increments from 1200 ms to 4000 ms (4 seconds); the default value is 1500 ms (1.5 seconds). The guard timer cannot be stopped manually.
The following example configures a guard timer value of 2000 ms.
device# configure terminal device(config)# erp 1 device(config-erp-1)# guard-time 2000
Use the no guard-time command to clear the configuration, restoring it to the default value.