Automatic Protection Switching of Packet-over-SONET Circuits

Feature Summary

Platforms

Supported MIBs and RFCs

Configuration Tasks

Configuration Examples

Command Reference

The protection mechanism used for this feature has "1+1, architecture" as described in the Bellcore publication TR-TSY-000253, SONET Transport Systems; Common Generic Criteria, Section 5.3. The connection may be bidirectional or unidirectional, and revertive or non-revertive.

In the 1+1 architecture, a protect interface (circuit) is paired with each working interface. Normally, the protect and working interfaces are connected to a SONET ADM (Add-Drop Multiplexer), which sends the same signal payload to the working and protect interfaces. The working and protect circuits may terminate in two ports of the same adapter card, or in different adapter cards in the same router, or in two different routers.

On the protect circuit, the K1 and K2 bytes from the line overhead (LOH) of the SONET frame indicate the current status of the APS connection and convey any requests for action. This signalling channel is used by the two ends of the connection to maintain synchronization.

The working and protect circuits themselves, within the router or routers in which they terminate, are synchronized over an independent communication channel, not involving the working and protect circuits. This independent channel may be a different SONET connection, or a lower-bandwidth connection. In a router configured for APS, the configuration for the protect interface includes the IP address of the router (normally its loopback address) 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, the process controlling the protect circuit directs the process containing the working circuit whether to activate or deactivate the working circuit, in the case of degradation or loss of channel signal, or manual intervention. If communication between the two processes is lost, the working router assumes full control of the working circuit as if no protect circuit existed.

In bidirectional mode, the receive and transmit channels are switched as a pair. In unidirectional mode, the transmit and receive channels are switched independently. For example, in bidirectional mode, if the receive channel on the working interface has a loss of channel signal, both the receive and transmit channels are switched.

In addition to the new Cisco IOS commands added for the APS feature, the POS interface configuration commands pos  threshold and pos report have been added to support user configuration of the bit error rate (BER) thresholds and reporting of SONET alarms.

For more information on POS interfaces, refer to the Cisco IOS documentation (in particular, the "Configuring Interfaces" chapter in the Configuration Fundamentals Configuration Guide), and the installation and configuration documentation that accompanies the POS hardware.

Perform these tasks in privileged EXEC mode to display the information described:

To display the current BER threshold setting or to view the reporting of the SONET alarms, use the show controllers pos EXEC command.

On router A, which contains the working interface, use the following configuration:

On router B, which contains the protect interface, use the following configuration:

To verify the configuration or to determine if a switchover has occurred, use the show aps command.

Syntax Description

Default

Authentication is disabled.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps authenticate command to ensure that only valid packets are accepted on the OOB communication channel.

The aps authenticate command must be configured on both the working and protect interfaces.

Examples

The following example enables authentication on POS interface 0 in slot 4:

Related Commands

Syntax Description

Default

No circuit is switched.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps force command to manually switch the interface to a protect interface when you are not using the aps revert command. For example, if you need to change the fiber connection, you can manually force the working interface to switch to the protect interface.

You can use the aps force 0 command to force traffic from the protect interface back onto the working interface. This is true only because APS always has one-plus-one (1+1) architecture.

The aps force command has a higher priority than any of the signal failures or the aps  manual command.

The aps force command is configured only on protect interfaces.

The aps force command takes effect immediately and is not saved with the configuration.

Examples

The following example forces the circuit on POS interface 0 in slot 3 (a protect interface) back onto a working interface:

Related Commands

Syntax Description

Default

No groups exist.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

The aps group command specifies a protect-group number for the protect or working circuit to which it applies.

The aps group command must be configured on both the protect and working interfaces.

The aps group command should appear before the aps working or aps protect command.

The aps group command specifies how the various working and protect circuits are associated. The aps group command must be configured on both the protect and working interfaces.

Examples

The following example configures two working/protect interface pairs. Working interface (3/0/0) is configured in group 10 (the protect interface for this working interface is configured on another router), and protect interface (2/0/1) is configured in group 20:

On the second router, protect interface (4/0/0) is configured in group 10, and working interface (5/0/0) is configured in group 20 (the protect interface for this working interface is configured on another router):

Related Commands

Syntax Description

Default

No lockout exists.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

The aps lockout command is configured only on protect interfaces.

Examples

The following example locks out (that is, prevents the circuit from switching to a protect interface in the event that the working circuit becomes unavailable) the POS interface 3/0/0:

Related Commands

Syntax Description

Default

No circuit is switched.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps manual command to manually switch the interface to a protect interface. For example, you can use this feature when you need to perform maintenance on the working channel. If a protection switch is already up, you can also use the aps manual command to revert the communication link back to the working interface before the wait to restore (WTR) time has expired. The WTR time period is set by the aps revert command.

In a one-plus-one (1+1) configuration only, you can use the aps manual 0 command to force traffic from the protect interface back onto the working interface.

The aps manual command is a lower priority than any of the signal failures or the aps force command.

Examples

The following example forces the circuit on POS interface 0 in slot 3 (a working interface) back onto the protect interface:

Related Commands

Syntax Description

Default

No circuit is protected.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Examples

The following example configures circuit 1 on POS interface 5/0/0 as a protect interface for the working interface on the router with the IP address of 7.7.7.7. For information on how to configure the working interface, refer to the aps working command.

Related Commands

Syntax Description

Default

Automatic switchover is disabled.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps revert command to return the circuit to the working interface when it becomes available.

The aps revert command is configured only on protect interfaces.

Examples

The following example enables circuit 1 on POS interface 5/0/0 to revert to the working interface after the working interface has been available for 3 minutes:

Related Commands

Syntax Description

Default

Hello time is 1 second, and hold time is 3 seconds.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps timers command to control the time between an automatic switchover from the protect interface to the working interface after the working interface becomes available.

The hold timer and hello timer control aspects of the OOB communications.

Normally, the hold time is greater than or equal to three times the hello time.

The aps timers command is configured only on protect interfaces.

Examples

The following example specifies a hello time of 2 seconds and a hold time of 6 seconds on circuit 1 on POS interface 5/0/0:

Syntax Description

This command has no keywords or arguments

Default

Bidirectional mode.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Use the aps unidirectional command when you must interoperate with SONET network equipment (ADMs) that supports unidirectional mode.

The aps unidirectional command is configured only on protect interfaces.

Examples

The following example configures POS interface 3/0/0 for unidirectional mode:

Syntax Description

Default

No circuit is configured as working.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

To enable the circuit on the protect interface to switch back to the working interface after the working interface becomes available again, use the aps revert interface configuration command.

Examples

The following example configures the POS interface 0 in slot 4 as a working interface. For information on how to configure the protect interface, refer to the aps protect command.

Related Commands

Syntax Description

This command has no keywords or arguments.

Default

No line alarm indication signal is sent.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC and Cisco IOS Release 11.2 GS.

In Automatic Protection Switching (APS) environments, AIS-L can be used to force a protection switch.

Prior to Release 11.2(11)GS2, when a POS interface shut down the framer was reset. In 11.2(11)GS2 and later 11.2GS releases, the framer no longer is reset when a POS interface is shut down. The pos  ais-shut interface configuration command controls whether an Alarm Indication Signal - Line (AIS-L) is sent when a POS interface is shut down.

It is possible for an interface to shut down silently (no alarm) if pos  ais-shut is not configured. For more information on APS, refer to Automatic Protection Switching of Packet-over-Sonet Circuits.

Example

The following example forces the alarm indication on the POS OC-3 interface 0 in slot 3:

Syntax Description

Default

b1-tca, b2-tca, b3-tca, plop, sf-ber, slof, and slos are reported by default.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Reporting an alarm means that the alarm can be logged to the console. Just because an alarm is permitted to be logged does not guarantee that it is logged. SONET alarm hierarchy rules dictate that only the most severe alarm of an alarm group is reported. Whether an alarm is reported or not, you can view the current state of a defect by checking the "Active Defects" line from the show controllers pos command output. A defect is a problem indication that is a candidate for an alarm.

For B1, the bit interleaved parity error report is calculated by comparing the BIP-8 code with the BIP-8 code extracted from the B1 byte of the following frame. Differences indicate that section level bit errors have occurred.

For B2, the bit interleaved parity error report is calculated by comparing the BIP-8/24 code with the BIP-8 code extracted from the B2 byte of the following frame. Differences indicate that line level bit errors have occurred.

For B3, the bit interleaved parity error report is calculated by comparing the BIP-8 code with the BIP-8 code extracted from the B3 byte of the following frame. Differences indicate that path level bit errors have occurred.

PAIS is sent by line terminating equipment (LTE) to alert the downstream path terminating equipment (PTE) that it has detected a defect on its incoming line signal.

PLOP is reported as a result of an invalid pointer (H1, H2) or an excess number of new data flag (NDF) enabled indications.

SLOF is detected when a severely error framing (SEF) defect on the incoming SONET signal persists for 3 milliseconds.

SLOS is detected when an all-zeros pattern on the incoming SONET signal lasts 19(+-3) microseconds or longer. This defect might also be reported if the received signal level drops below the specified threshold.

To determine the alarms that are reported on the interface, use the show controllers pos command.

Example

The following example enables reporting of SD-BER and LAIS alarms on the interface:

Related Commands

Syntax Description

Default

The default is 6 for b1-tca, b2-tca, b3-tca, and sd-ber. The default is 3 for sf-ber.

Command Mode

Interface configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

For B1, the bit interleaved parity error report is calculated by comparing the BIP-8 code with the BIP-8 code extracted from the B1 byte of the following frame. Differences indicate that section level bit errors have occurred.

For B2, the bit interleaved parity error report is calculated by comparing the BIP-8/24 code with the BIP-8 code extracted from the B2 byte of the following frame. Differences indicate that line level bit errors have occurred.

For B3, the bit interleaved parity error report is calculated by comparing the BIP-8 code with the BIP-8 code extracted from the B3 byte of the following frame. Differences indicate that path level bit errors have occurred.

SF-BER and SD-BER are sourced from B2 BIP-8 error counts (as is B2-TCA). However, SF-BER and SD-BER feed into the APS machine and can lead to a protection switch (if APS is configured).

B1-TCA, B2-TCA, and B3-TCA do nothing more than print a log message to the console (if reports for them are enabled).

To determine the BER thresholds configured on the interface, use the show controllers pos command.

Example

The following example configures thresholds on the interface:

Related Commands

Syntax Description

This command has no arguments or keywords.

Command Mode

Privileged EXEC

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC.

Sample Displays

The following is sample output of the show aps command on a router configured with a working interface. In this example, POS interface 0/0/0 is configured as a working interface in group 1, and the interface is selected (that is, active).

The following is sample output of the show aps command on a router configured with a protect interface. In this example, POS interface 2/0/0 is configured as a protect interface in group 1, and the interface is not selected (the ~ indicates that the interface is not active). The output also shows that the working channel is located on the router with the IP address 15.1.6.1 and that the interface is currently selected (that is, active).

Syntax Description

Command Mode

Privileged EXEC

Usage Guidelines

This command first appeared in Cisco IOS Release 11.1 CC and 11.2 GS.

The information displayed is generally useful for diagnostic tasks performed by technical support personnel only.

Sample Display

The following is sample output of the show controllers pos command on a Cisco 7500 series router: