Table Of Contents
Configuring Automatic Protection Switching on the Cisco ASR 903 Router
Automatic protection switching (APS) is a protection mechanism for SONET networks that enables SONET connections to switch to another SONET circuit when a circuit failure occurs. A protect interface serves as the backup interface for the working interface. When the working interface fails, the protect interface quickly assumes its traffic load.
This document contains the following sections:
Understanding Automatic Protection Switching
The protection mechanism used for this feature is "1+1, Bidirectional, nonrevertive" as described in the Bellcore publication "TR-TSY-000253, SONET Transport Systems; Common Generic Criteria, Section 5.3." In the 1+1 architecture, there is one working interface (circuit) and one protect interface, and the same payload from the transmitting end is sent to both the receiving ends. The receiving end decides which interface to use. The line overhead (LOH) bytes (K1 and K2) in the SONET frame indicate both status and action.
The protect interface is configured with the IP address of the router that has the working interface. The APS Protect Group Protocol, which runs on top of UDP, provides communication between the process controlling the working interface and the process controlling the protect interface. Using this protocol, interfaces can be switched because of a router failure, degradation or loss of channel signal, or manual intervention. In bidirectional mode, the receive and transmit channels are switched as a pair.
Two SONET/SDH connections are required to support APS. In a telco environment, the SONET/SDH circuits must be provisioned as APS. You must also provision the operation (for example, 1+1), mode (for example, bidirectional), and revert options (for example, no revert). If the SONET/SDH connections are homed on two separate routers (the normal configuration), an out of band (OOB) communications channel between the two routers needs to be set up for APS communication.
When configuring APS, we recommend that you configure the working interface first. Normal operation with 1+1 operation is to configure it as a working interface. Also configure the IP address of the interface being used as the APS OOB communications path.
APS uses Protect Group Protocol (PGP) between working and protect interfaces. The protect interface APS configuration should include an IP address of a loopback interface on the same router to communicate with the working interface using PGP. Using the PGP, POS interfaces can be switched in case of a degradation or loss of channel signal, or manual intervention. In bidirectional mode, the receive and transmit channels are switched as a pair.
In bidirectional APS the local and the remote connections negotiate the ingress interface to be selected for the data path. The egress interface traffic is not transmitted to both working and protect interfaces.
Inter Chassis Redundancy Manager
ICRM provides these capabilities for stateful MLPPP with MR-APS Inter-Chassis Redundancy implementation:
•Node health monitoring for complete node, PE, or box failure detection. ICRM also communicates failures to the applications registered with an ICRM group.
•Reliable data channels to transfer the state information.
•Detects active RP failure as node failure and notifies the controllers.
ICRM on the standby RP re-establishes the communication channel with peer node if the active RP fails.
For instructions on how to configure ICRM, see Configuring Stateful MLPPP with MR-APS Inter-Chassis Redundancy.
The following limitations apply when using APS on the Cisco ASR 903 Router
•APS is not currently supported with CES.
•APS is not currently supported with ATM.
•APS is not currently supported with IMA.
•APS supports HDLC, PPP, and MLPPP encapsulation.
•ATM Layer 2 AAL0 and AAL5 encapsulation types are supported
•APS is only supported on MLP and serial interfaces on the OC-3 interface module.
Configuring Automatic Protection Switching Interfaces
The following sections describe how to configure APS interfaces:
Note We recommend that you configure the working interface before the protected interface in order to prevent the protected interface from becoming the active interface and disabling the working interface.
Note For information about configuring optical interfaces for the first time, see the Cisco ASR 903 Series Router Chassis Configuration Guide.
Configuring a Working Interface
To configure a working interface, use the following commands beginning in global configuration mode.
1. controller sonet slot / port-adapter / port
2. aps group group-number
3. aps working circuit-number
Configuring a Protect Interface
To configure a protect interface, use the following commands beginning in global configuration mode.
5. controller sonet slot / port-adapter / port
6. aps protect circuit-number ip-address
Configuring Other APS Options
To configure the other APS options, use any of the following optional commands in interface configuration mode.
Configuring Stateful MLPPP with MR-APS Inter-Chassis Redundancy
The Cisco ASR 903 Router supports Stateful MLPPP with Inter-Chassis Redundancy. For information on how to configure this feature, see http://www.cisco.com/en/US/docs/ios/wan/configuration/guide/wan_mlppp_mr_aps.html.
Monitoring and Maintaining APS
To provide information about system processes, the Cisco IOS software includes an extensive list of EXEC commands that begin with the word show, which, when executed, display detailed tables of system information. Following is a list of some of the common show commands for the APS feature.
To display the information described, use these commands in privileged EXEC mode.
For more information about these commands, see the Cisco IOS Interface and Hardware Component Command Reference.