This document provides a brief overview of Packet over SONET (POS)
automatic protection switching (APS) technology.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
The APS feature is supported on Cisco 7500 Series Routers and Cisco
12000 Series Routers. This feature allows switchover of POS circuits in the
event of circuit failure and is often required when you connect SONET equipment
to Telco equipment. Automatic Protection Switching (APS) refers to the
mechanism of using a protect POS interface in the SONET network as the backup
for a working POS interface. When the working interface fails, the protect
interface quickly assumes its traffic load. Based on the configuration, the two
circuits can be terminated in the same router, or in different routers.
The protection mechanism used for this feature has a 1+1 architecture
as described in the Bellcore publication TR-TSY-000253, SONET Transport
Systems; Common Generic Criteria, Section 5.3. The connection can be
bi-directional 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
can 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 signaling channel is used by the two ends
of the connection to maintain synchronization.
The working and protect circuits are synchronized themselves within
each router or routers in which they terminate over an independent
communication channel. This channel does not involve direct communication
through the working and protect circuits. This independent channel can 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
The APS Protect Group Protocol, which runs on top of User Datagram
Protocol (UDP), provides communication between the process that controls the
working interface and the process that controls the protect interface. In the
case of degradation, loss of channel signal, or manual intervention, the
process that controls the protect circuit sends commands to the process that
contains the working circuit and activates or deactivates the working circuit
as necessary. If communication between the two processes is lost, the working
router assumes full control of the working circuit as if no protect circuit
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® Software commands added for the APS
feature, the POS interface configuration commands pos
threshold and pos report are added in
order to support user configuration of the bit error rate (BER) thresholds and
reporting of SONET alarms.
These examples show how to configure basic APS on a router and how to
configure more than one protect/working interface on a router by using
the aps group command.
This example shows the configuration of APS on router A and router B
(see diagram). In this example, router A is configured with the working
interface, and router B is configured with the protect interface. If the
working interface on router A becomes unavailable, the connection automatically
switches over to the protect interface on router B.
Use this configuration on router A, which contains the working
router(config)#interface loopback 1
router(config-if)#ip address 220.127.116.11 255.255.255.0
router(config)#interface pos 2/0/0
router(config-if)#aps group 1
router(config-if)#aps working 1
Use this configuration on router B, which contains the protect
router(config)#interface loopback 2
router(config-if)#ip address 18.104.22.168 255.255.255.0
router(config)#interface pos 3/0/0
router(config-if)#aps group 1
router(config-if)#aps protect 1 22.214.171.124
Note: The loopback interface is used as the interconnect. The APS group
command is used even when a single protect group is configured.
Use the show aps command in order to verify
the configuration or to determine if a switchover has occurred.