Automatic Protection Switching Configuration


Note

Automatic Protection Switching is not supported on the Cisco ASR 900 RSP3 module.

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.

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 Stateful MLPPP Configuration with MR-APS Inter-Chassis Redundancy, page 5.

Limitations

  • Starting Cisco IOS XE Release 3.11, APS is supported with CES.

  • The APS group number range supported on the RSP2 module in aps group group-number acr command is 1-191.

  • APS is not supported with ATM.

  • APS is not supported with IMA.

  • APS is not supported with POS.

  • 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.

  • You can perform a force switchover to overcome the following limitations:

    • An interface module OIR, either through physical or software causes the APS switchover time to be higher (500-600ms).

    • Shut or no shut of the port may lead to higher APS switchover time.

    • Removal of the active working or protect interface may lead to higher APS switchover time.

Automatic Protection Switching Interfaces Configuration

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.

Before you begin

To configure the controller in SDH mode, see Configuring Optical Interface Modules.

Procedure

  Command or Action Purpose

Step 1

controller sonet slot / port-adapter / port

Example:


Router(config)# controller sonet 0/0/0

Returns to controller configuration mode.

Step 2

aps group group-number acr

Example:


Router(config-if)# aps group acr 1

Configures the working interface group on a router. The APS group number must be greater than 1.

Step 3

aps working circuit-number

Example:


Router(config-if)# aps working 1

Configures this interface as a working interface. 1 is the only supported circuit-number value.

Step 4

end

Example:


Router(config-if)# end

Exits configuration mode.

Configuring a Protect Interface

To configure a protect interface, use the following commands beginning in global configuration mode.

Before you begin

To configure the controller in SDH mode, see Configuring Optical Interface Modules.

Procedure

  Command or Action Purpose

Step 1

controller sonet slot / port-adapter / port

Example:


Router(config)# controller sonet 0/0/0

Returns to controller configuration mode.

Step 2

aps group group-number acr

Example:


Router(config-if)# aps group acr 2

(Optional) Allows more than one protect/working interface group to be supported on a router.

Step 3

aps protect circuit-number ip-address

Example:


Router(config-if)# aps protect 1 7.7.7.7

Configures the interface as a protect interface and specifies the IP address of the device that contains the working interface.

Step 4

end

Example:


Router(config-if)# end

Exits configuration mode.

Configuring Other APS Options

To configure the other APS options, use any of the following optional commands in interface configuration mode.

Procedure

  Command or Action Purpose

Step 1

aps authenticate string

Example:


Router(config-if)# aps authenticate authstring

(Optional) Configures the authentication string that the router uses to authenticate PGP message exchange between protect or working routers. The maximum length of the string is eight alphanumeric characters. Spaces are not accepted.

Step 2

aps force circuit-number

Example:


Router(config-if)# aps force 1

(Optional) Manually switches the specified circuit to a protect interface, unless a request of equal or higher priority is in effect. For example, if the protect interface is configured as circuit 1, use the aps force 1 command to set the protect interface to active.

Note

 

If you do not want the protect port to be active all the time, use no aps force 1 command after using aps force 1 command. Similarly for aps force 0 use use no aps force 0 command.

Step 3

aps group group-number

Example:


Router(config-if)# aps group 2

(Optional) Allows more than one protect/working interface group to be supported on a router.

Step 4

aps lockout circuit-number

Example:


Router(config-if)# aps lockout 1

(Optional) Prevents a working interface from switching to a protect interface. For example, if the protect interface is configured as circuit 1, use the aps lockout 1 command to prevent the protect interface from becoming active.

Step 5

aps manual circuit-number

Example:


Router(config-if)# aps manual 0
(Optional) Manually switches a circuit to a protect interface, unless a request of equal or higher priority is in effect. For example, if the working interface is configured as circuit 0, the command is applied as follows:

  • The aps manual 0 command activates the working interface

  • The aps manual 1 command activates the protect circuit.

Applying the no form of the command removes the configuration and stops the router from sending K 1and K2 bytes on the interface.

Step 6

aps revert minutes

Example:


Router(config-if)# aps revert 10

(Optional) Enables automatic switchover from the protect interface to the working interface after the working interface becomes available.

Step 7

aps timers seconds1 seconds2

Example:


Router(config-if)# aps timers 1 5
(Optional) Specifies the following values:

  • seconds1 —The time between hello packets.

  • seconds2 —The time that the working interface can be down before the router switches to the protect interface.

Step 8

aps unidirectional

Example:


Router(config-if)# aps unidirectional

(Optional) Configures a protect interface for unidirectional mode.

Example


Router# configure terminal
Router# interface gigabit ethernet 0/1/0
Router(config-if)# aps force 1
Ruter(config-if)# aps unidirectional

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.

  • Use the show aps command to display information about APS.

  • Use the show controller sonet slot command to display information about the controller port.

  • use the show interfaces command to display information about the interface.

For more information about these commands, see the Cisco IOS Interface and Hardware Component Command Reference.