Hi Byron,
What yo have said is true for most part of it. let me explain a little more.
Soft preemption attempts to establish a new path for a preempted LSP before tearing down the original LSP. The default behavior is to tear down a preempted LSP first, signal a new path, and then reestablish the LSP over the new path. In the interval between when the path is taken down and the new LSP is established, any traffic attempting to use the LSP is lost. Soft preemption prevents this type of traffic loss. The trade-off is that during the time when an LSP is being soft preempted, two LSPs with their corresponding bandwidth requirements are used until the original path is torn down.
Another use case for soft-preemption can be during network-maintenances, where you want to move all LSPs away from a particular link, and then take the interface down without interrupting traffic. You can find more details on soft-preemption in RFC 5712.
As you already said, soft-preemption is useful to minimize traffic interruption where Superior priority LSP pushes other LSPs to new links.But when we are taking about the other 3 of the 4 scenarios like LSP reroute due to some link failure, explicit-path modification or LSP re-optimization then the useful knob would be adaptive.
You can configure an LSP to be adaptive when it is attempting to reroute itself. When it is adaptive, the LSP holds onto existing resources until the new path is successfully established and traffic has been cut over to the new LSP. It avoids double-counting BW for the links that share the new and old paths as it works on Shared Explicit reservation style, which means the sessions share single bandwidth reservation.
To achieve hitless switchover for LSPs (Make before break) you can leverage both these knobs along with some addiitonal features.
As for the best practices to achieve MBB for LSPs, kindly refer the below techlibrary document,
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/make-before-break-achieving-for-lsps.html
I hope this helps !
Regards,
Ankur
JNCIE x 3 (SP,DC & ENT)
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