Switching Mode Overview
There are two types of switching mode:
- Unidirectional mode
- Bidirectional mode
You can configure interoperation with SONET/SDH Line Terminating Equipment (LTE) that is provisioned for unidirectional linear APS in 1+1 architecture on the following interfaces:
- Unchannelized OC3, OC12, and OC48 SONET/SDH interfaces on T Series routers
- SONET/SDH interfaces on the M40e router
- ATM over SONET interfaces
By default, APS supports only SONET/SDH LTE that is provisioned for bidirectional mode.
In bidirectional switching mode, the working interface switches to the protect interface for both receipt and transmission of data, regardless of whether the signal failure is in the transmit or receive direction.
In true unidirectional mode, the working interface switches to the protect interface only for the direction in which signal failure occurs; for example, if there is a signal failure in the transmit direction, the working interface switches over to the protect interface for transmission but not receipt of data. When the protect interface operates in unidirectional mode, the working and protect interfaces must cooperate to operate the transmit and receive interfaces in a bidirectional fashion.
The Junos OS does not support true unidirectional mode. Instead the software supports interoperation with SONET/SDH LTE provisioned for unidirectional switching. This means that the SONET/SDH LTE on the router receives and transmits on one interface, even when you configure unidirectional support.
The Junos implementation of unidirectional mode support allows the router to do the following:
- Accept a unidirectional mode as valid
- Trigger the peer (ADM) selector to switch receive from working interface to protect interface or the reverse
- Not send reverse requests to the far end (ADM)
