Specifications

Configure Automatic Protection Switching of Packet-over-SONET Circuits

IC-100

Cisco IOS Interface Configuration Guide

Conﬁgure an Alarm Indication Signal

To conﬁgure line alarm indication signals (LAIS) when the POS interface is placed in any

administrative shut down state, use the following command in interface conﬁguration mode:

Conﬁgure Automatic Protection Switching of Packet-over-SONET

Circuits

The automatic protection switching (APS) feature is supported on Cisco 7500 series routers. This

feature allows switchover of packet-over-SONET (POS) circuits and is often required when

connecting SONET equipment to telco equipment. APS refers to the mechanism of bringing a

“protect” POS interface into the SONET network as the “working” POS interface on a circuit from

the intervening SONET equipment.

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, POS interfaces can be switched due to 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. In unidirectional mode, the transmit and receive channels are switched independently. For

example, 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.

APS Conﬁguration Task List

Two SONET connections are required to support APS. In a telco environment, the SONET 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 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 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.

For more information on POS interfaces, refer to the installation and conﬁguration documentation

that accompanies the POS hardware.

Command Purpose

pos ais-shut Send line alarm indication signals.