Configuring SONET Automatic Protection System

This chapter contains a description and configuration information for the SONET Automatic Protection System (APS), which can be used to provide line and card redundancy for BXM OC-3 and OC-12 cards.

Refer to the Cisco WAN Switching Command Reference for further information on configuration and monitoring commands.

Introduction

Automatic Protection Switching (APS) configures a pair of SONET lines for line redundancy so that hardware automatically switches from a Working line to a Protection line when the Working line fails, and vice versa, within a specified period after an active line failure.

Each redundant line pair consists of a Working Line and a Protection Line. The concept of Working and Protection Lines is similar to the concept of Primary and Secondary Y Redundant cards. That is, the Working line is the logical line to which the user refers.

Left undisturbed, hardware performs line switching automatically. Upon detection of a Signal Fail condition (LOS, LOF, Line AIS or Bit Error Rate exceeding a configured limit) or a Signal Degrade condition (BER exceeding a configured limit), hardware switches from the Working Line to the Protection Line (assuming the Working line was the Active line and the Protection line is not in alarm).

If the "Revertive" option is enabled, (cnfapsln command), the hardware automatically switches back to the working line from the protection line after a configured time period called "Wait to Restore" (cnfapsln command) has elapsed. The working line must be in a clear state for this to occur. The revertive option is the default for APS 1:1 but not for APS 1+1.

Coordination between the two ends of the line is accomplished using the in-band protocol.

During setup, the commands addapsln, cnfcdaps, and cnfapsln are used to create the line-redundant pair. Also, appropriate front cards, back cards, and a special RDNT-BP daughter backplane are required for APS 1+1 configurations.

During operation, signal failure or signal degradation can cause APS "switchovers". A switchover is when the line that was active gives up control to its partner line. This partner line now becomes the "active" line, while the original active line becomes the "standby" line.

Implementation for BXM Cards

Automatic Protection Switching provides a standards based line-redundancy for BXM OC-3 and OC-12 cards. With Release 9.2, the BXM OC-3 and BXM OC-12 cards support the SONET APS 1+1 and APS 1:1 standards for line redundancy which is provided by switching from the working line to the protection line.

The working line is normally the active line, and the protection line is normally the standby line.

The APS 1+1 and APS 1:1 protocols that are supported by the BXM are listed in Table 25-1 and shown in Figure 25-2 and Figure 25-3, respectively. APS 1+1 Annex B has the same general layout as shown in Figure 25-2, except that the active line is called the primary, and the standby line is referred to as the secondary.

Table 25-1 BXM SONET APS
Protocol	Description
APS 1+1	The APS 1+1 redundancy provides card and line redundancy, using the same numbered ports on adjacent BXM back cards.
APS 1:1	The APS 1:1 redundancy provides line redundancy, using adjacent lines on the same BXM back card.
APS 1+1 Annex B	The APS 1+1 Annex B redundancy provides 1+1 high-speed protection, which can be configured only for bidirectional, nonrevertive protection switching. For Annex B, the active line is referred to as the "primary section" and the standby line is referred to as the "secondary section". Manual switching (switchapsln) is not allowed in the APS 1+1 Annex B implementation.

Tiered Management Control

SONET is defined across three elements, section, line, and path as shown in Figure 25-1 and described in Table 25-2. An advantage of this tiered approach is that management control can be exercised at each level, for example at the section level independent of the line or path level.

Note APS on the BPX requires the use of single mode fiber, not multi-mode fiber, on both ends of the trunk. The PXM cards on any connected MGXs should also be single mode fiber.

Table 25-2 SONET Section, Line, and Path Descriptions
Unit	Description
Section	A section is the fiber optic cable between two active elements such as simple repeaters. The active element terminating these sections is called Section Terminating Equipment (STE).
Line	A line is a physical element that contains multiple sections and repeaters and is terminated by line terminating equipment (LTE) at each end.
Path	A path includes sections and lines and terminates at the customer premises equipment (CPE).

Table 25-3 provides a cross-reference between OC-n optical carrier levels, the equivalent STS-n and SDH-n levels, and the associated line rates.

Table 25-3 Digital Hierarchies
OC-n Optical Carrier	STS-n Synchronized Transport Signal	Line Rates (Mbps)	SDH-n Synchronized Digital Hierarchy	STM-n Synchronous Transport Module
OC-1	STS-1	51.84
OC-3	STS-3	155.52	SDH-1	STM-1
OC-12	STS-12	622.08	SDH-4	STM-4
OC-48	STS-48	2488.32	SDH-16	STM-12

Manual Operation

SONET Automatic Protection Switching configures a pair of SONET lines for line redundancy so that the interface hardware automatically switches from a working line to the protection line or vice versa within a specified period after an active line failure.

However, you may use the switchapsln command to manually control switching. The last user switch request (switchapsln) per line pair is saved by switch software so that the APS can be configured correctly in the event of a node rebuild.

Operation Criteria

APS cards provide both front and back card LED displays providing line and card status active and standby status.

APS Front Card Displays

APS 1+1 LED Displays

The back cards used for APS 1+1 with front card redundancy have an LED which indicates whether the back card can be pulled out for service replacement.

Table 25-4 BXM Front Card LED Display
LED	Description
Card LED, Green	Active
Card LED, Yellow	Inactive
Port LED, Green	Line is active
Port LED, Yellow	Line is standby

For example, all the lines on the card except one may be working properly and therefore the card needs to be replaced. The back card LED functions are listed in Table 25-5.

Note In the APS 1+1 configuration, when the primary card is active and the protection line is active, LEDs on both back cards are green. The LED of the secondary is green because that back card is carrying traffic. The LED of the primary back card is green, because that is in the physical path of the front card in receiving traffic from the protection line. When the back card LED is green do not pull out the back card, because it will disrupt traffic. When the LED is yellow it is OK to pull out the back card, but it should be put back as soon as possible, because the card is needed in the event of a switchover.

APS 1+1 (Card and Line Redundancy)

Table 25-5 BXM Back Card for APS 1+1 LED Display
LED	Description
Green	The card has at least one active line and may not be removed without affecting service.
Yellow	The card has no active lines and may be removed.
Red	Not used and not applicable.

The APS 1+1 feature requires two BXM front cards, an APS redundant frame assembly, and two redundant type BXM back cards. The two redundant BXM back cards are plugged into the APS redundant frame assembly as shown in Figure 25-4. The following are the types of available back cards:

Each of the listed model numbers includes two single back cards and one mini-backplane (providing cross coupling of two back cards).

Figure 25-4 APS 1+1 Redundancy, Installing APS Back Cards in APS Redundant Backplane

Traffic protected by APS 1+1 redundancy is carried via the working line and the protection line simultaneously (see Figure 25-5). Bridging is implemented such that the same payloads are transmitted identically over the working line as the protection line.

The receiver terminating the APS 1+1 has to select cells from either the working or protection line and be able to forward one consistent traffic stream. Since both working and protection line transport identical information, the receiving ends can switch from one to the other without the need for coordinating with the transmit end.

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The addapsln command defines which line is working and which is protection.

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Before you can execute the addapsln command for a line pair, the protection line must be in the standby state.

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If the addapsln command is executed, the working line is always initially selected.

When no port on a BXM is configured for APS, each back card of the pair may be used independently by independent front cards. The switch software disallows configuration of APS if independent usage is detected. There must be no active lines on the card that is selected to be the secondary card.

With previous card cages, because of the positioning of mechanical dividers, the APS card pairs can only be inserted in certain slots. These are slots 2 through 5 and 10 through 13. The mechanical dividers are located at slots 1 and 2, 5 and 6, 9 and 10, and 13 and 14.

With current card cages, this limitation is removed, and the APS card pairs can be located anywhere, except BCC cards slots 7 and 8, and ASM card slot 15.

An APS 1+1 redundant card pair must be in adjacent slots (2,3 or 4,5 and so on).

APS 1+1 Redundancy Criteria

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The two BXM front cards reside in the same two adjacent slots as the APS back cards

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The working lines on the back card must be connected to the same slot as the primary front card

The switching of the front cards is controlled by switch software under the Y-redundancy protocol. The switch software performs switching between the two cards in the event of a front card failure, front card downed, front card failing self-test, and so on.

You may add APS at any time after Y-redundancy is configured as long as the protection line is in the standby state. You may add APS even if lines and trunks are upped and the card is passing traffic.

Note Normally when APS and card redundancy are implemented together, the term YRED really means card redundancy, as in this case there is no Y-cabling involved. An exception exists when the BXM is attached to a MGX 8220 (feeder shelf) or other device which does not support APS. In that case, Y-cables or straight cables may be used with APS.

When APS is configured on a card pair, switch software checks to ensure that both cards match and support APS.

For APS 1+1 redundancy, the same numbered ports on adjacent BXM back cards are used. The maximum number of connections supported does not change, as the complete connection capability of the cards is available.

Note Using only one front card and two back cards is not a valid configuration when adding APS capability, and the APS alarm capability is reduced when the standby card is not available.

Application Notes for APS 1+1

Using switchcdred/switchyred command

Note Entering switchcdred or switchyred execute the same command. The newer name is switchcdred which replaces switchyred, but switchyred may still be used for those familiar with that command.

The switchcdred (switchyred) command can be used to switch between an active and standby front card in an APS 1+1 configuration. For example, you might want to do this to test the standby front card.

Following a switchcdred (switchyred), or active card reset, the BXM card is sent a message from switch software to have it perform an APS switch to align itself with the last user switchapsln switch request.

If the last user request is "clear", full automatic APS switching is in effect with the working line in the active state by default. When there is no last user switch request to switch any particular line, for example, protection line, the working line becomes active.

Note In the APS 1+1 configuration, if the protection line is active and the last user request is "clear," a switchdred causes the working line to be active if there is no line condition on working line. When APS 1+1 comes up, it comes up on the working line if the working line is clear. When a switchcdred is issued, the active card also comes up on the working line if the working line is clear and there is no user request. In the case where the working line is in alarm or there is a user request to switch to the protection line (switchapsln), the card will first come up on the working line. Then the card will detect the alarm or the user request and switch to the protection line.

Notes on switchcdred

Note In the APS 1+1 configuration, if the last user request was a W -> P switch, then dsplog will log a W -> P switching event when a switchcdred is issued. On a switchcdred, the newly active card comes up on working line first. Then it responds to a user request to switch from working to protection by switching to the protection line and sending an event notification to that effect. The event notification can be seen in the event log by using the dsplog command.

Note It may be necessary to perform a switchcdred (switchyred) command after performing a service switch with the switchapsln command so that the back card that the service switch selects has its associated front card active.

Notes on switchapsln

With APS 1+1, when repetitive switchapsln commands are issued, up to two in a row can be executed sequentially, when alternating between options 3 and 4 (forced switch), or 5 and 6 (manual switch), but no more. Attempts to execute a third switchapsnln will not succeed, and the following error message is displayed:

If you wish to perform repetitive switchapsln commands, you should issue a clear switch between each W-P, P-W pair of commands, for example:

Configuring APS 1+1

Step 1

Verify that appropriate front and back cards are installed along with APS two-card daughterboard.

Step 2

Ensure that lines are connected, for example on port 1 of BXM card in slot 2 and port 1 of BXM card in slot 3.

Step 3

Execute these commands and verify chan half= no, and standard= GR-253 (default)

Step 4

Execute the following command, for example, for redundant line on port 1 for BXM OC-3 cards and APS back cards in slots 2 and 3 of the BPX:

Step 5

addapsln 2.1 3.1 1 {addapsln<slot.port> <slot.port> <1|2|3|..>

Note

The last entry, "1", in the addapsln command specifies the type of APS, in this example APS 1+1.

APS 1:1 (Line Redundancy)

The APS 1:1 feature provides port and line redundancy for a single BXM front card and associated OC-3 or OC-12 redundant back card.

There is no new hardware required to support APS 1:1. A single front card with a standard back card is used.

Two adjacent lines on the same card are used. The maximum number of connections supported by a nonenhanced BXM card is reduced by half for APS 1:1 operation. Using enhanced BXM cards, the number of available connections is not decreased.

Similarly to APS 1+1, SONET APS 1:1 requires that for every working line, there must exist a redundant protection line (see Figure 25-6). However, unlike the 1+1 case, traffic protected by the redundancy must be carried on the protection line only when a failure occurs on the working line. In the case of no failure, the protection line can transport idle traffic, `same' traffic as working line, or extra traffic. Because the protection line is not guaranteed to carry real traffic until the transmit end is informed of the failure and switches, this coordination between the equipment at both ends is more complex.

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Before the addapsln command is used, the switch software will not attempt to use or monitor the protection line; only the working line is used.

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If the addapsln command is used with a working line in place, the working line is always initially selected.

General Criteria

APS 1:1 cannot be configured on cards already configured for YRED. They cannot be configured concurrently. Use APS 1 + 1 instead.

APS 1:1 configuration requires that the user add the APS configuration to a line before upping the line.

APS 1:1 configuration requires that the user down a line prior to deleting the APS configuration on the line.

APS 1:1 can only be configured for bidirectional operation and revertive switching.

Configuration Criteria

The redundant lines must be adjacent. In addition, the lines which may be paired are:

Either of the two lines may be designated as working line and the other as the protection line.

The switching of the working and protection lines is controlled by BXM firmware/hardware under the APS protocol.

The BPX firmware/hardware performs switching between the protection and working lines in the event of a line or port failure.

The user may add APS as long as the working and protection line are in the standby state. Lines and trunks can only be upped after APS 1:1 is added.

Configuring APS 1:1

Note Before configuring for APS 1:1 redundancy, all card connections must be deleted using the delcon command

Step 1

Ensure that lines are connected, for example on ports 1 and 2 of a BXM in slot 3.

Note

The last entry, "2", in the addapsln command specifies the type of APS, in this example APS 1:1.

Step 2

Execute cnfcdaps and verify chan half= yes (not default), and standard= GR-253 (default)

Step 3

addapsln 3.1 3.2 2 {addapsln<slot.port> <slot.port> <1|2|3|4|5>

APS 1 +1 Annex B Card and Line Redundancy

The APS 1 +1 Annex B feature is similar to the APS 1+1 feature, with the main difference being that APS 1+1 Annex B redundancy only can be configured for bi-directional operation and nonrevertive switching.

General Criteria

APS 1 + 1 Annex B can be configured only for bidirectional operation and nonrevertive switching on a line.

Note In nonrevertive switching, to avoid data loss, a line is not automatically switched back to active after a failure is corrected.

Configuring APS 1+1 Annex B

Step 1

Verify that appropriate front and back cards are installed along with APS two-card daughterboard.

Step 2

Ensure that lines are connected, for example port 1 on BXM in slot 1 and port 1 on BXM in slot 2.

Step 3

Execute the following commands and verify chan half= no, and standard= GR-253 (default):

Step 4

Execute the following command, for example, for redundant line on port 1 for BXM OC-3 cards and APS back cards in slots 1 and 2 of the BPX:

Step 5

addapsln 1.1 2.1 3 {addapsln<slot.port> <slot.port> <1|2|3|..>

Note

The last entry, "3", in the addapsln specifies the type of APS, in this example APS 1 + 1, Annex B.

Test Loops

The test commands addlnloclp and addlnrmtlp may affect service even when APS is configured. In all APS configurations, if the working line is looped, both lines are looped and traffic disrupted.

Notes on APS Messages

When adding an APS 1+1 line or trunk by using addapsln, if the working slot's paired redundant slot is not a legal protection slot, or if firmware cannot determine what the paired slot is, an invalid slot pairing exists and one of the following two messages are displayed:

You can display the redundant card information by using the dspcd command under the "Backcard Installed" heading. For example, if a redundant pair is configured with a primary slot of 2 and a secondary slot of 3, the dspcd 2 command displays RedSlot: 3, and the dspcd 3 command displays RedSlot: 2.

APS K1 Command Precedence

The possible conditions which may cause/prevent a switch are listed in Table 25-6. The list is arranged starting from highest precedence and ending with lowest precedence. For further description and information, refer to the Cisco WAN Switching Command Reference.

Table 25-6 K1 Switching Conditions
APS K1 Command Precedence
Lock out of Protection
Forced Switch
Signal Fail
Signal Degrade
Manual Switch
Wait To Restore
Reverse Request
Do not Revert
No Request

APS Command Summary

Commands to support APS are listed in Table 25-7, and defined in more detail in the Cisco WAN Switching Command Reference where you will find thorough information on each of these commands, as well as other commands not listed here.

Table 25-7 APS Commands
Name	Description
New Commands Added for Management of APS
cnfcdaps slot	Sets APS options on the card.
addapsln slot1.port1 slot2.port2 protocol	Adds APS.
delapsln slot.port	Deletes APS.
dspapsln	Displays status of APS line pairs.
switchapsln slot.port (option 1...6, S)	Controls the APS user switching interface.
cnfapsln slot.port	Configures the APS parameters on a line.
New Commands for Card Redundancy for APS 1+1
addcdred	Adds redundancy across two cards.
dspcdred	Displays the redundant cards.
delcdred	Deletes redundancy configuration for cards.
switchcdred	Switches active and redundant cards.
Commands modified for use with APS
cnfbkcd	Specifies that the APS options are modified.
dspalms	Adds a row for "APS Alarms" that lists Minor and Major APS alarms.
dspcd	Displays front and back card APS attributes. For the front card, displays that card supports APS 1+1 and APS 1:1. For the back card, displays if back card is a redundant back card, and if so, the slot number of the redundant back card. Also, displays APS mismatch conditions.
dspsv3	Displays APS alarms pending.
dsplog	Displays APS alarms.
Commands modified for use with APS (continued)
addyred	Specifies that the addyred command is modified to prevent invalid configurations when combined with APS.
delyred	Specifies that the delyred command is modified to prevent invalid configurations when combined with APS.

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