Using APS to Protect Against SONET Circuit Failures

Problem

One of your routers has two SONET interfaces that connect to the same add/drop multiplexer (ADM). You want to make sure that if one of the PICs or FPCs fails, the router does not lose its connection with the ADM.

Solution

To set up basic APS, first configure the working circuit:

	[edit 
interfaces so-3/1/0 sonet-options]
	aviva@t320# set aps working-circuit APS-at-my-colo
	aviva@t320# set aps authentication-key $1991poPPi

Then configure the protect circuit:

	[edit interfaces so-1/1/0  sonet-options]
	aviva@t320# set aps protect-circuit APS-at-my-colo 
	aviva@t320# set aps authentication-key $1991poPPi 

 

Discussion

APS allows a SONET circuit to switch over to a backup (protect) circuit in the event that the active (working) circuit fails. If the working circuit fails or degrades, the ADM and protect router switch the traffic to the protect circuit, which becomes active. JUNOS software uses APS 1+1 switching, which pairs a working circuit with a protect circuit. It supports both revertive and nonrevertive modes, but only bidirectional mode. The APS specification ( GR-253-CORE, SONET Transport Systems: Common Generic Criteria) requires that the working and protect circuits transmit identical data, but the JUNOS software does not do this, which turns out to have no operational impact.

When protecting the PIC or FPC, configure two SONET interfaces on different FPCs. In this recipe, the working circuit is on FPC3 and the protect circuit is on FPC1. The APS group name, here APS-at-my-colo, and the authentication key must be the same to associate the two interfaces. On the ADM side, the router's working circuit must be connected to the ADM's working circuit, and the router's protect circuit must likewise be connected to the ADM's protect circuit.

When using APS to protect the entire router, use the same configuration on SONET interfaces in two different routers and also specify the address of the other router:

	[edit interfaces so-3/1/0 sonet-options]
	aviva@t320# set aps neighbor 192.0.8.2/24

Use the show aps command to check that the APS circuits are up:

	aviva@t320>  
show aps
	Interface Group Circuit Intf state
	so-1/1/0 APS-at-my-colo Protect disabled, up
	so-3/1/0 APS-at-my-colo Working enabled, up

The output shows that in the group APS-at-my-colo, the working circuit, so-3/0/1, is operational and the backup circuit, so-1/1/0, is down. You can also see this in the show interfaces command output:

	aviva@t320> show interfaces so-1/1/0
	Physical interface: so-1/1/0,  
Administratively down, Physical link is Up
	…
	 Logical interface so-1/1/0.0 (Index 67) (SNMP ifIndex 63)
	 Flags: Hardware-Down Device-Down Point-To-Point SNMP-Traps
	 Encapsulation: PPP
	 Protocol inet, MTU: 4470
	 Flags: Protocol-Down
	 Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
	 Destination: 192.0.2/24, Local: 192.0.2.2, Broadcast: 192.0.2.255

This output shows that the physical link is up, but the physical interface is administratively down because you have configured it as a backup. The logical interface is also down.

The show aps group command gives detailed status of the two circuits:

	aviva@t320> show aps group APS-at-my-colo
	Interface Group Circuit Intf state
	so-1/1/0 APS-at-my-colo Protect disabled, up
	Neighbor 0.0.0.0, adj up,neighbor interface enabled,dead 2.912
	 Channel state Working
	 Working circuit is on interface so-3/1/0
	local-mode bidirectional(5),neighbor-mode bidirectional(5)
	 Req K1 0x00, rcv K1 0x00, xmit K1 0x00,nbr K1 0x00, nbr paired req 0
	 Revert time 0, neighbor revert time 0
	 Hello due in 0.632
	so-3/1/0 APS-at-my-colo Working enabled, up
	Neighbor 0.0.0.0, adj up,neighbor interface disabled,dead 2.672
	 Channel state Working
	 Protect circuit is on interface so-1/1/0
	local-mode bidirectional(5),neighbor-mode bidirectional(5)
	 Req K1 0x00, rcv K1 0x00, xmit K1 0x00,nbr K1 0x00, nbr paired req 0
	 Revert time 0, neighbor revert time 0
	 Hello due in 0.861

By default, APS is nonrevertive, which means that if the protect circuit becomes active, traffic is not switched back to the working circuit unless the protect circuit fails or you manually configure a switch to the working circuit. You can set the circuit to switch back after a specified amount of time, here 15 minutes:

	[edit interfaces so-3/1/0  sonet-options]
	aviva@t320# set aps revert-time 9000 

The ADM must also be in revertive mode. You can also manually switch the circuit back to being the working circuit:

	[edit interfaces so-3/1/0  sonet-options]
	aviva@t320# set 
aps request working 

 

See Also

GR-253-CORE, SONET Transport Systems: Common Generic Criteria