Configuring SONET APS

Introduction

SONET Automatic Protection Switching (APS) provides for the rapid failover of traffic from a working line to a dedicated protection line, based on Telcordia GR-253-CORE and ANSI T1.105-2000. The IPriori implementation is a bidirectional, non-reverting, linear, 1+1, APS architecture that allows for the designation of pairs of working and protected lines in both a simple or distributive configuration. A clearing of the working line failure will not cause a switchover back (non-reverting) unless the user so commands, or a subsequent failure occurs on the protection line.

Simple and distributive configurations are supported. In a simple configuration both working and protective lines originate from the same router, though not necessarily from the same linecard. The SONET APS Channel protocol selects the working line. In a distributive configuration the working and protective lines originate from different Avici routers, that uses an IP network connection between the two routers. Line viability is achieved via Hello packets sent at the expiration of each keepalive period. The keepalive does not require a direct point-to-point connection between the routers, though a direct connection does optimize the failover scenario.

An APS interface pair can be on the same module, across different modules on the same router, or, using the distributive configuration, across different Avici routers. A module can be configured for a mixture of APS and non-APS interfaces. In the simple configuration, Avici routers can be at both ends or one end can be a third party router. Working and protected lines may be directly connected to SONET ADM or be directly connected between two routers.

APS is supported on ATM interfaces. A set of ATM subinterfaces is configured on the working line and mirrored on the protection line. It is the users responsibility to assure that the configuration of sub-interfaces on both the working and protection lines are the same. Only one line is active at any one time. If the active line goes down, the mirrored inactive line becomes active.

The APS implementation supports directly reachable host address IP prefixes belonging to the IP subnet that are forwarded or transmitted over an APS interface. The ATM port must be administratively shutdown when enabling or disabling APS. When switching from non-APS to an APS mode on an ATM interface no atm ilmi-enable and no atm ilmi-keepalive are assumed and enforced by the interface configuration. For an interface transition from non-APS to APS mode the following sub-interface configurations are assumed and enforced: no ilmi manage, no oam-pvc manage, no oam-retry. Assurance of PVC configuration compatibility between the protection and working interfaces is the responsibility of the user.

A working line can be configured and activated without the presence of mirrored a protection line. The protection line can be configured while the working line is active, but will not take over for a failed working line until fully configured.

The following topics are covered in this chapter:

• Configuring authentication string for APS messages
  
  
• Initializing a forced or manual switch between working and protection interfaces
  
  
• Configuring an APS working and protection interface group
  
  
• Configuring a working to protection lockout
  
  
• Enabling working or protection mode on an interface
  
  
• Configuring the keepalive and holddown timers for APS.
  
  

Configuring the APS Authentication String

A string of up to 8 characters can be configured for the authentication of communications between the working and protection routers. A communications link protocol is used by both the simple and distributed APS configurations for communications between working and protected interfaces. Authentication must be enabled or changed prior to the APS protection interface becoming operational. Authentication is not required by APS.

Use the aps authenticate command to configure an authentication string for communications between the working and protection routers by APS.

Example: In the following example the ATM interface 1/1/1 is:

• Administratively shutdown using the shutdown command
  
  
• Assigned to the bostonNYC APS group
  
  
• Configured to be the working APS interface
  
  
• Assigned an APS authentication string of auth1
  
  
• Enabled using the no shutdown command
  
  

The ATM interface 1/1/2 is:

• Administratively shutdown using the shutdown command
  
  
• Assigned to the bostonNYC APS group
  
  
• Configured to be the protection APS interface
  
  
• Assigned an APS authentication string of auth1
  
  
• Enabled using the no shutdown command
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps working 1 router(config-if)#aps authenticate auth1 router(config-if)#no shutdown router(config-if)#exit router(config)#interface atm 1/1/2 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps protect 1 router(config-if)#aps authenticate auth1 router(config-if)#no shutdown router(config-if)#end router#

Configuring an APS Group

The group name binds the working and protection interfaces on a router. In the case of a distributed APS configuration, the group name must be the same, and unique within an APS distributive domain, on both routers that make up the working and protection interface pair. If a group name is not specified for an APS interface pair, it defaults to 0. Multiple APS working and protection pairs on a single router must be uniquely identified with a group name.

The ATM interface must be administratively down when configuring a group name. The group name can be configured either before or after the APS interface is identified as a working or protection line. If the group name is configured after the interface is put in APS mode, it implies a change in the group name from either 0 or another configured group name to the new group name.

The interface does not enter APS mode when the group name is specified. APS mode is entered using the aps working or aps protect commands. When multiple groups are configured on a router, it is highly recommended that the group name be specified before an interface enters APS working or protection mode to avoid conflicts with an already existing default group 0.

Use the aps group command to configure an interface with a group name.

The recommended procedure is to:

1. Administratively shutdown the interface using the shutdown command.
   
   
2. Configure group name using the aps group command.
   
   
3. Enable working or protection mode on the interface using the aps working or aps protect command.
   
   
4. Enable the interface using the no shutdown command.
   
   
5. Repeat the procedure for the second interface in the pair.
   
   

Example: In the following example the ATM interface 1/1/1 is:

• Administratively shutdown using the shutdown command
  
  
• Assigned to the bostonNYC APS group
  
  
• Configured to be the working APS interface
  
  
• Enabled using the no shutdown command
  
  

The ATM interface 1/1/2 is:

• Administratively shutdown using the shutdown command
  
  
• Assigned to the bostonNYC APS group
  
  
• Configured to be the protection APS interface
  
  
• Enabled using the no shutdown command
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps working 1 router(config-if)#no shutdown router(config-if)#exit router(config)#interface atm 1/1/2 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps protect 1 router(config-if)#no shutdown router(config-if)#end router#

Configuring an APS Mode

An APS interface pair consists of one interface enabled for the active working mode that carries the SONET payload and a second interface enabled for passive protection mode. The protection interface passively stands by ready to transmit the SONET payload should the user perform an APS interface switch or should a failure occur on the working interface.

APS working and protection modes are enabled in interface configuration mode for each interface of the APS pair. The interface must be administratively shutdown before enabling an APS mode.

Each APS interface pair belongs to an APS group. If no APS group is specified for the interface, the interface defaults to group 0. Only a single interface pair can be configured for group 0. It is highly recommended that an interface be assigned to a group unique to the APS interface pair prior to enabling APS mode on the interface to avoid multiple interface pairs enabled for group 0.

When configuring both the working and protection interfaces, the working interface circuit number can be specified. In the current implementation, the working interface circuit number is always 1. Specifying the working interface circuit number is currently optional. The protection interface circuit is always 0.

There are two possible APS configurations: simple and distributive. The distributive mode requires that a communications line exist between the protection interface on one router and the working interface on a second router for the APS implementation. In distributive configuration, the working circuit number and an IP address of an interface on the router containing the working interface must be specified when configuring the APS protection interface. Avici recommends that a loopback address on the router containing the working interface be used.

Use the aps working command in interface configuration mode to configure a port for APS working mode.

Use the aps protect command in interface configuration mode to configure a port for APS protection mode.

Example 1: The following example:

• Enters configuration mode for ATM interface 1/1/2.
  
  
• Administratively shuts down the interface.
  
  
• Configures the interface as a member of APS group bostonNYC.
  
  
• Configures the interface for protection mode and specifies IP address 10.10.10.01 as being a valid interface on the router upon which the working interface resides.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/2 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps protect 0 10.10.10.01 router(config-if)#no shutdown router(config-if)#exit router#

Example 2: In the following example:

• Enters configuration mode for ATM interface 1/1/1/.
  
  
• Administratively shuts down the interface.
  
  
• Configures the interface as a member of APS group bostonNYC.
  
  
• Configures the interface for working mode.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#shutdown router(config-if)#aps group bostonNYC router(config-if)#aps working 1 router(config-if)#no shutdown router(config-if)#exit router#

Switching Between APS Interfaces

There are times when it is desirable to voluntarily switch traffic between the APS working interface and the protection interface, for instance, when needing to perform maintenance on the working interface. IPriori provides two commands for switching between APS interfaces that are differentiated by the level of priority for the switch. A manual switch will be executed if a failure, a lockout, or a force switch condition is not in affect on the interface the traffic will be moved to. If any of these three conditions exist, the manual switch will not take place. A forced switch will take place as long as a protection interface lockout condition is not in affect.

A manual or forced switch can be performed by specifying either the circuit number of the APS mode to switch from: 0 specifies a switch from the protection to the working interface, 1 specifies a switch from the working to the protection interface, or by specifying the keyword for the APS mode to switch to: protect specifies a switch to the protection interface from the working, working specifies a switch to the working from the protection interface.

The interface switch can be performed from either the working or protection interface configuration mode, and will be ignored if entered from a non-APS interface.

Use the aps manual command to manually switch between working and protective APS interfaces.

Use the no aps manual command to cancel a pending manual switch that has already been issued. Canceling a manual switch does not necessarily cause an immediate switch back to the previous interface. The switch back will occur if and when an appropriate switch causing condition presents itself.

Example: The following example:

• Enters interface configuration mode for the 1/1/1 ATM interface.
  
  
• Issues a manual switch command to the APS protection interface from the working interface.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#aps manual protect router(config-if)#end router#

Use the aps force command to force a high priority switch between working and protective APS interfaces.

Use the no aps force command to cancel a pending interface switch that has already been issued. Canceling an interface switch does not necessarily cause an immediate switch back to the previous interface. The switch back will occur if and when an appropriate switch causing condition presents itself.

Example 1: The following example:

• Enters interface configuration mode for the 1/1/1 ATM interface.
  
  
• Issues a forced interface switch command to the APS protection interface for this working interface.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#aps force protect router(config-if)#end router#

Example 2: The following example:

• Enters interface configuration mode for the 1/1/1 ATM interface.
  
  
• Issues a force switch command to the APS protection interface from the working interface.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#aps force 1 router(config-if)#end router#

Lockout APS Protection Mode Switch

Switching to protection mode can be locked out under all circumstances. If the protection interface is in use when this request is made, a switch is made back to the working interface regardless of its state. This command overrides any outstanding APS manual or forced interface conditions that may be present.

Unlike the aps manual and aps force commands, the aps lockout command is part of the running-config of the interface on which it is issued and is only available from the protection interface.

Use the aps lockout command to prevent a switch from the working to protection interface under all conditions.

Example: The following example:

• Enters configuration mode for the ATM 1/1/1 protection interface
  
  
• Performs a shutdown of the interface
  
  
• Enables lockout for this interface
  
  
• Enables the interface using the no shutdown command
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#shutdown router(config-if)#aps lockout router(config-if)#no shutdown router(config-if)#end router#

Configuring APS Timers

There are two APS timers: the time between hello packets which is set by the keepalive-time and the time between the sending a hello packet and its response on the working interface. Hello packets are sent at the expiration of each keepalive-time. When the other end of the link receives a hello packet it returns a response. If the response is not received before the expiration of the holddown-time, the link is declared dead. These timers are valid for both the simple and distributive APS configurations.

Timer values have a granularity of 16.667 milliseconds and can be expressed as a floating point value. Specified values are rounded up.

The holddown time defaults to 3 times the keepalive-time configuration or can be optionally specified.

Use the aps timers helloVal command to specify a keepalive-time and default the holddown-time to 3 times the specified keepalive-time.

Use the aps timers helloVal holdVal command to specify both the keepalive-time and holddown-time.

Example 1: The following example:

• Enters ATM interface mode using the interface atm command for interface 1/1/1.
  
  
• Configures the keepalive-time for 1.25 seconds and defaults the holddown-time to 3.75 seconds.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#aps timers 1.25 router(config-if)#end router#

Example 2: The following example:

• Enters ATM interface mode using the interface atm command for interface 1/1/1.
  
  
• Configures the keepalive-time for 1.25 seconds and the holddown-time to 3 seconds.
  
  

router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)#interface atm 1/1/1 router(config-if)#aps timers 1.25 3.00 router(config-if)#end router#

Displaying APS Statistics

The following commands are available for displaying APS behavior and status.

Table 7-1. IS-IS Show Commands  

Show Command	Description
show aps brief	Displays a single line of information identifying name, associated interface, circuit type, and state information for each group.
show aps detail	Displays detailed status per group for this router.
show aps interface	Displays detailed statistics for the specified interface.
show aps group	Displays detailed statistics for the specified group.

Copyright © 2005 Avici Systems Inc.
Avici® and TSR® is a registered trademark of Avici Systems Inc.
IPriori™, Composite Links™, SSR™, QSR, and NSR® are trademarks of Avici Systems Inc.

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