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Configuring SONET/SDH Defect Hold Times
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
- 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
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 you configure hold timers and the interface
goes from up to down, the interface transition is not advertised to
the rest of the system until the interface has remained down for the
hold-time period. Similarly, when an interface goes from down to up,
the interface transition is not advertised until the interface has
remained up for the hold-time period.
To configure hold timers, include the hold-time statement at the [edit interfaces interface-name sonet-options trigger defect] hierarchy
- [edit interfaces interface-name sonet-options
hold-time up milliseconds down milliseconds;
The time can be a value from 1 through 65,534 milliseconds.
When you configure defect hold times, you should
note the following:
- You can configure an up hold time, a down hold time, or
- 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
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 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
- Configure a combined up hold time and down hold time for
a SONET/SDH defect to be at least 100 milliseconds.
Example: Configuring SONET/SDH
Defects to Be Ignored
Prevent an LOS from bringing down an interface.
An LOS can lead to the following defects:
- [edit interfaces sonet-options trigger]
- ais-l ignore;
- lof ignore;
- los ignore;
- pll ignore;
- rfi-l ignore;
- rfi-p ignore;
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