Switching Between the Working and Protect Circuits Overview
When there are multiple reasons to switch between the working and protect circuits, a priority scheme is used to decide which circuit to use. The routers and the ADM might automatically switch traffic between the working and protect circuits because of circuit and router failures. You can also choose to switch traffic manually between the working and protect circuits.
When an ATM2 PIC is configured for APS, and the protect circuit comes online for the first time, there are no open VCs and the PIC discards the input traffic received on the protect circuit. The show interface extensive or show monitor interface traffic operational mode commands display the statistics as zero since the PIC drops the packets at the VC.
When the APS switches from the working circuit to the protect circuit, VCs are created on the protect circuit to accept traffic. However, the VCs on the working circuit remain open to support any future APS switches even though the interface is down or disabled. The input traffic received on the working circuit (current backup) is accepted by the PIC but discarded in the PFE. The show interface extensive or show monitor interface traffic operational mode commands displays live statistics for the traffic since it is accepted by the PIC.
When APS switches from the protect circuit to the working circuit again, the VCs on the protect circuit remain open to support a future APS switch even though the interface is down or disabled. The input traffic received on the current backup protect circuit is accepted by the PIC but discarded in the PFE. The show interface extensive or the show monitor interface traffic operational mode command displays live statistics for this traffic since it is accepted by the PIC.
There are three priority levels of manual configuration, listed here in order from lowest to highest priority:
Request (also known as manual switch)—Overridden by signal failures, signal degradations, or any higher-priority reasons.
Force (also known as forced switch)—Overrides manual switches, signal failures, and signal degradation.
Lockout (also known as lockout of protection)—Do not switch between the working and protect circuits.
Do not use the disable statement at the [edit interfaces interface-name aps] hierarchy level to switch between interface working and protect circuits; it can cause loss of traffic on the disabled interface. Use only the request statement or the force statement at the [edit interfaces interface-name aps] hierarchy level to modify interface status.
A router failure is considered to be equivalent to a signal failure on a circuit.
M120 routers and M320 routers with Enhanced III FPCs support Annex B lockout.
The lockout feature is supported as follows:
The selector position will be at what it was before the lockout feature was configured (no switching of working and protect circuits).
Transmitted K1/K2 will be frozen (same K1 and K2 bytes will be transmitted as before the lockout).
The APS will ignore requests from the peer to switch working and protect circuits.
For Annex B, lockout must be configured on both local and remote ends, as they are not signaled using K1/K2 bytes as in a non Annex B configuration.