SONET/SDH Defect Hold Times for Damping Interface Transitions Overview
By default, an interface is marked down as soon as a defect is detected, and is marked up as soon as the defect is absent. You might want to apply hold times to defects for the following reasons:
To prevent route flaps from happening before a defect has been outstanding for a longer period than would be expected for an Automatic Protection Switching (APS) cutover
To reduce the number of interface transitions
On M Series and T Series routers with Channelized SONET IQ PICs and Channelized SONET IQE PICs, the SONET defect alarm trigger hold-time statement is not supported.
When you apply a “down” hold time to a defect, the defect must be present for at least the hold-time period before the interface is marked down. When you apply an “up” hold time to a defect, the defect must remain absent for at least the hold-time period before the interface is marked up, assuming no other defect is outstanding.
When a hold-down timer is configured and the interface goes from up to down, the down hold-time timer is triggered. Every interface transition that occurs during the hold-time is ignored. When the timer expires and the interface state is still down, then the router begins to advertise the interface as being down. Similarly, when a hold-up timer is configured and an interface goes from down to up, the up hold-time timer is triggered. Every interface transition that occurs during the hold-time is ignored. When the timer expires and the interface state is still up, then the router begins to advertise the interface as being up.
When you configure defect hold times, you should note the following:
You can configure an up hold time, a down hold time, or both.
Each interface on a SONET/SDH PIC controls certain aspects of the SONET/SDH overhead. For example, when you configure an OC48 PIC to be nonconcatenated, four interfaces are created. Each interface has its own path overhead. However, all four path interfaces share the same physical, section, and line overhead. This means the following:
Each interface’s path trigger configuration is honored.
The physical, section, and line trigger configuration for the primary interface (so-fpc/pic/slot:0) is applied to all four interfaces.
Therefore, if you configure the so-fpc/pic/slot:0 interface to have a hold time for the LOS trigger, when an LOS event occurs, all four interfaces remain up until the trigger expires, and then all four interfaces are marked down.
The hold timers on the SONET/SDH defects are applied in addition to any other hold timers you configure on the interface. For example, if an interface is up and you configure a SONET/SDH trigger down hold time of 100 milliseconds and an interface down hold time of 250 milliseconds, when the SONET/SDH defect occurs, the SONET/SDH trigger timer starts. After 100 milliseconds, assuming the defect is still present, the SONET/SDH defect starts the 250 millisecond down timer. After this has expired and again assuming the defect is still outstanding, the interface will be marked down. For more information about interface hold timers, see Damping Shorter Physical Interface Transitions.
Some defects are reported through a periodic poll (once every second). For these defects, there could be up to one second lost before the defect is detected and the hold timer is started. The hold timer expires in precisely the amount of time configured. At that point, the existence of the defect is checked again and the interface is marked up or down accordingly. These defects are as follows:
We recommend the following settings:
Configure SONET/SDH defect timers on no more than 64 interfaces per FPC.
Configure a combined up hold time and down hold time for a SONET/SDH defect to be at least 100 milliseconds.