Traditional APS is configured on two independent physical SONET interfaces: one configured as the working circuit and the other as the protect circuit (see Figure 1). The circuit, named Circuit X in the figure, is the link between the two SONET interfaces.
Figure 1: APS Interface
Traditional APS uses routing protocols that run on each individual SONET interface (since circuit is an abstract construct, instead of being an actual interface). When the working link goes down, the APS infrastructure brings up the protect link and its underlying logical interfaces, and brings down the working link and its underlying logical interfaces, causing the routing protocols to reconverge. This consumes time and leads to traffic loss even though the APS infrastructure has performed the switch quickly.
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