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Automatic Protection Switching and Multiplex Section Protection Overview


Automatic Protection Switching (APS) is used by SONET add/drop multiplexers (ADMs) to protect against circuit failures. The Junos implementation of APS allows you to protect against circuit failures between an ADM and one or more routers, and between multiple interfaces in the same router. When a circuit or router fails, a backup immediately takes over.


For SDH interfaces, the Junos OS supports multiplex section protection (MSP). You configure MSP with the same CLI statements you use to configure APS.

The Junos OS supports APS 1+1 switching, either revertive or nonrevertive mode, and bidirectional mode only (although you can configure interoperation with line-terminating equipment [LTE] provisioned for unidirectional mode). The Junos OS does not transmit identical data on the working and protect circuits, as the APS specification requires for 1+1 switching, but this causes no operational impact.

For DS3 channels on a channelized OC12 interface, you can configure APS on channel 0 only. If you configure APS on channels 1 through 11, it is ignored.

With APS and MSP, you configure two circuits, a working circuit and a protect circuit. Normally, traffic is carried on the working circuit (that is, the working circuit is the active circuit), and the protect circuit is disabled. If the working circuit fails or degrades, or if the working router fails, the ADM and the protect router switch the traffic to the protect circuit, and the protect circuit becomes the active circuit.

To configure APS or MSP, you configure a working and a protect circuit, as shown in Figure 1. To protect against a router failure, you connect two routers to the ADM, configuring one of them as the working router and the second as the protect router. To protect against a PIC or FPC failure, you connect one router to the ADM through both the working and protect circuits, configuring one of the PICs or FPCs as the working circuit and the second as the protect circuit.

Figure 1: APS/MSP Configuration Topologies
APS/MSP Configuration

This implementation of APS is not supported on Layer 2 circuits. For Layer 2 circuits, configure APS by including the protect-interface statement. You can include this statement at the following hierarchy levels:

  • [edit logical-systems logical-system-name protocols l2circuit neighbor neighbor-id interface interface-name]

  • [edit protocols l2circuit neighbor neighbor-id interface interface-name]

For more information and a configuration example, see the Junos OS VPNs Library for Routing Devices.

When configuring the APS annex-b option, the APS options must be configured as follows:

  • switching-mode cannot be uni-directional

  • revert-time cannot be configured

  • fast-aps-switch cannot be configured

  • lockout is allowed to be configured

  • wait-to-restore-time is allowed only when Annex-B is configured

  • protect-circuit must be configured

  • working-circuit must be configured