In Active/Standby pseudowires a failed pseudowire has recovered. Under what circumstances will the active pseudowire revert to the previously failed…

In Nokia's Service Router Operating System (SR OS), Active/Standby pseudowire redundancy uses a precedence value to determine the active path. The path with the lowest numerical precedence is preferred. The default behavior is revertive, meaning if the primary (more preferred) path fails and later recovers, traffic will automatically switch back to it. A path is designated as primary by assigning it the lowest possible precedence value, which is 0. The keyword primary is a direct alias for the precedence value 0. Therefore, the service will revert to the previously failed link if that link was configured as the primary path.

A. It will always revert to the previously failed link.

This is incorrect. Revertive behavior is the default but can be disabled. For instance, if both pseudowires have the same precedence, the system will not revert upon recovery.

B. It will never revert to the previously failed link.

This is incorrect. Revertive switching is the default and standard operational mode for an Active/Standby configuration where one link is designated as primary.

D. It will revert if the precedence of the previously failed link is lower than the currently active link.

While this statement describes the general mechanism of precedence-based switching, option C is more precise. The term "revert" specifically implies returning to a designated primary path, which is explicitly configured with precedence 0 or primary.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VLL services"

Section: Pseudowire Redundancy

Quote/Content: "The system always chooses the spoke-SDP with the lowest precedence value to be the active spoke-SDP... The precedence command allows for a spoke-SDP to be configured as a primary, secondary, or tertiary path... The keyword primary is an alias for a precedence value of 0... If the primary spoke-SDP fails, the system automatically switches to the secondary spoke-SDP. When the primary spoke-SDP is restored, the system reverts to the primary." This directly links the concept of "reverting" to the recovery of the "primary" path, which is defined by precedence 0.