APS Load Sharing Between Circuit Pairs Overview
When two routers are connected to a single add/drop multiplexer (ADM), you can have them back up each other on two different pairs of circuits. This arrangement provides load balancing between the routers if one of the working circuits fails.
Figure 1 illustrates load sharing between circuits on two routers. Router A has a working circuit “Start” and a protect circuit “Up,” and Router B has a working circuit “Up” and a protect circuit “Start.” Under normal circumstances, Router A carries the “Start” circuit traffic and Router B carries the “Up” circuit traffic. If the 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.”