Juniper JUNOS 10.1 - CONFIGURATION GUIDE 1-2010 Configuration Manual page 995

Chapter 61: Configuring SONET/SDH Interfaces
working circuit "Start" were to fail, Router B would end up carrying all the traffic for
both the "Start" and "Up" circuits.
To balance the load between the circuits, you pair the two circuits. In this case, you
pair the "Start" and "Up" circuits. Then, if the working circuit "Start" fails, the two
routers automatically switch the "Up" traffic from the working to the protect circuit
so that each router is still carrying only one circuit s worth of traffic. That is, the
working circuit on Router A would be "Up" and the working circuit on Router B would
be "Start."
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Configuring SONET/SDH Physical Interface Properties