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This chapter describes the process for configuring the SONET interfaces in both Path-Protection Switching (PPS) and Linear Automatic Protection Switching (LAPS) mode operation. In addition, autoprovisioning and loopback support are described.
Prior to configuring the SONET interfaces, you should determine the application for which the Cisco Optical Networking System (ONS) 15303 will be deployed. For example, ring versus linear applications and protected versus unprotected applications. In protected ring applications, the Cisco ONS 15303 provides PPS down to the VT1.5 level. In protected linear, also known as terminal applications, LAPS is used to provide protection switching at the line level. In unprotected applications, no protection switching is performed.
The following commands are used to configure SONET interfaces:
When the Cisco ONS 15303 is deployed on an existing ring, minimal configuration of the OC-3 port is required. In that scenario, the OC-3 port and the STS-1 must be set to in-service. If no other network element exists on the ring, the OC-3 port(s) must be administratively set to in-service. When this is completed, the STS-1 can be configured. To set the OC-3 port(s) to in-service, at the config prompt, enter ine enter oc3 ab timing loop admin is.
To bring the OC-3 interface up, the OC-3 range must be specified (these values are a, b, or ab) and the is (in-service) command must be entered. Although all other parameters have default values that are generally acceptable, attention should be given to the timing configuration. In networks where clocking can be traced to a more reliable source than a stratum 3 clock, loop timing should be selected. If clocking is being provided by the Cisco ONS 15304, the default value, system, should be acceptable as the clocking source. Under most circumstances, the frc-ssm-msg option is also acceptable using the default setting of none. This setting permits the OC-3 to be used as the timing source. If the OC-3 is not to be the source of clocking on the ring, this parameter should be changed to DUS.
Configuring the STS-1 consists of specifying the STS-1 id number, which on the Cisco ONS 15303 is always 1, the path-type (drop and terminate is the only valid option), and administrative state (in-service or out-of-service). The STS-1 are auto-provisioned through the system when the OC-3 interface is configured. To configure the STS-1, at the config prompt, enter ine enter sts1 1 path0type drop admin is.
Configuring the VT1.5(s) consists of specifying the STS-1 id number (1), the VT1.5 id number (1 to 3), the OC-3 range {a | b | ab} the path-type (drop and terminate or pass-thru), and the administrative state (in-service or out-of-service). If the path-type is drop, the administrative service must be in-service. To configure the VT1.5, at the config prompt, enter ine enter vt1.5 1/1-3/1-7/1-3/ab path-type drop admin is. Repeat this for all VT1.5s that need to be entered.
Path Protection Switching (PPS) provides path level protection as per ITU-T G.841. It provides the following functions:
On the Cisco ONS 15303, when a cross-connect is set up between a PTP and an DS1 interface, a selection function has to continuously determine whether data received on the East or West path of the optical interface has to be sent on to the interface. When the Cisco ONS 15303 is configured to be in ring mode, PPS is enabled.
PPS makes the selection decision based on the integrity of data received on each of the optical paths and the specific path selection commands and parameters specified by the user. PPS follows a list of priorities specified in ITU-T G.841 to determine the path to be selected.
The integrity of the data received is determined by monitoring the following defects on each of the PTPs:
The order of priority (from highest to lowest) in determining whether a path switch should occur is:
1. Lockout of protection (user command)
2. Forced switch (user command)
3. Path hardware failure
4. Path out of service (user command)
5. Path defect (indication from driver)
6. Signal fail (indication from FM)
7. Signal degrade (indication from FM)
8. Manual switch (user command)
9. Wait-to-restore (user specified parameter)
The list implies that if a user command is entered for a path, the PPS module will go through the list of conditions that currently apply to each of the two paths and determine, based on this list of priorities, whether a path switch is required.
You are required to configure one of the two paths as the preferred path (also called working path) and the other path as the alternate path (protection path). You have to configure the cross-connect to be in revertive or non-revertive mode. Revertive mode implies that when the two paths are equally good, the preferred path is selected. Additionally, if the preferred path goes down and the alternate path is selected, the system will try to revert to the preferred path when the preferred path is just as good as the protection path or better. If revertive mode is selected, the user also has to configure a wait-to-restore timer (300 seconds to 7200 seconds). The wait-to-restore timer is the duration that the system has to wait before switching back to the preferred path when the preferred path becomes eligible to be the active path.
In revertive mode, because traffic is selected from the preferred path whenever possible, only three commands are valid:
The highest priority command, lockout of protection, is used when the alternate path should never be used as the source of traffic. The forced switch from preferred path command is used when traffic from the preferred path cannot be used due to maintenance work. The forced switch condition will force traffic to be selected from the alternate path even when the traffic has defect conditions. The manual switch from the preferred path is used when the user would like to select the alternate path (as long as traffic on it is of good quality) but would want the system to switch back to the preferred path if the alternate path has any defect conditions.
In non-revertive mode, though the preferred/alternate (or working/protection) nomenclature is still used, the two paths are treated as peers. Therefore, if the alternate path is the source of traffic, a path switch to the preferred path will not occur when the preferred path becomes equally eligible to carry traffic. Because the two paths are treated as peers, the commands that are applicable on both paths are the same (forced switch and manual switch). The forced switch stays in effect even when defects are present while the manual switch is superseded when defects are detected.
If a signal fail or signal degrade condition is received on the active path, and running the above list of priorities determines that the other path is more eligible to be the active path, PPS will cause a path switch. In revertive mode, after a path switch has occurred to the alternate path, if the signal fail or signal degrade indication clears on the preferred path, a wait-to-restore timer will be started. If the preferred path does not revert to a signal fail or signal degrade condition and there is no new user command before the wait-to-restore timer expires, the PPS module will switch the active path back to the preferred path. In non-revertive mode, clearing the signal fail or signal degrade indication on the preferred path will not normally result in a path switch.
The following conditions are logged by the PPS module to indicate changes in the state of the cross-connect for which the PPS module is providing the path level protection.
The Cisco ONS 15303 system configuration can be set to either terminal mode or ring mode. PPS is active in ring mode. Ring mode is enabled in the following situations:
The following commands are used in PPS. For more information on these commands, refer to the Cisco Optical Networking System 15304 Command Reference Guide.
Linear Automatic Protection Switching (LAPS) provides the multiplex section protection for traffic and provides the following functions:
LAPS module supports linear 1+1 architecture on the Cisco ONS 15303 platform when operating in terminal mode. In this mode, interface A is considered to be the working interface and interface B is the protection interface. There are a number of configuration parameters when operating in this mode:
(a) Unidirectional (default)---Switching occurs at the equipment node where the facility failure is detected independent of the selection at the other end.
(b) Bidirectional---Switching occurs simultaneously via the LAPS bytes at both equipment nodes (local and adjacent) causing both ends to select the same interface for dropping traffic.
(c) Non-revertive (default)---When the working interface becomes available for selection while the protected interface is still capable of dropping traffic, no switching back to the working interface is performed.
(d) Revertive switching (user-provisionable)---Unlike the non-revertive mode, the selector switches back to the restored working interface after an operator specified wait-to-restore time during which the working interface remains available for selection.
The following commands are used in LAPS. For more information on these commands, refer to the Cisco Optical Networking System 15304 Command Reference Guide.
The three types of sources that can cause a protection switch are signal defects of line A and B, LAPS requests, and operator requests:
This module logs a number of standing and transient conditions for the protected OC-3-n entity. The first four are valid only in the bi-directional switching mode.
Autoprovisioning allows you to put a Cisco ONS 15303 in a ring network and restore the ring protection for the original network without manual intervention. The feature restores the path-level ring by setting up all the equipped paths in pass-thru mode through the Cisco ONS 15304. The following restrictions apply to the autoprovisioning feature.
The following entities can be auto-provisioned. Autoprovisioning of these entities will only occur when the parent entity is provisioned for the first time and is in an in-service, normal state.
Two types of loopbacks are currently being supported for all SONET entities:
Facility loopback is supported for OC-3, STS-1, and VT1.5. Loopback functionality supported can be defined as follows:
Loopback support for SONET PTPs is provided only after the pointer processing and path overhead byte processing is completed. Because of this, transmit path overhead and pointer generator blocks in the path of the looped signal take place. This results in special handling of the administrative state and the defect detection in the receive stream when a loopback is setup.
Unequipped code is normally transmitted for an entity with an out-of-service administrative state. However, when an entity in this state is looped, the transmission of the unequipped code is stopped and the received signal is looped back.
An RDI maintenance signal is transmitted upstream when a defect is received. However, when a loopback is set, no defect is transmitted back for the defects detected from the path overhead bytes. When an AIS or LOP defect is detected from the pointer processing of the received stream, AIS is transmitted downstream and then gets looped back.
Posted: Fri Apr 7 10:27:43 PDT 2000
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