Preparing for IE test
for MPLS, there are two knobs: Soft-preemption and Adaptive
They seem to have very similar effects - smooth transition of LSP traffic, holding the old LSP up while transitioning traffic to the new LSP. A number of scenarios set this off: optimize-timer, connectivity interruption, preemption & path modification
according to their respective Techlibrary pages the only difference seems to be that bw is double counted with SP and not with adaptive. Though SP seems to only mention preemption as a trigger (as in a superior priority LSP pushes our LSP onto new links) - this seems clear with the name but hoping for confirmation here. I feel like I am missing something - are there any other differences? Do these four scenarios trigger either/both mechanisms? Any best practices involved?
"adaptive" sets LSP reservation style as Shared Explicit, JUNOS default is Fixed Filter
Both are described in RFC 3209 https://tools.ietf.org/html/rfc3209
"Make-before-break" for the SAME individual LSP is the default behaviour and is not configurable. However, it is not the default for SEVERAL LSPs which need to yield each other to fit into a given finite network BW to be established and that's where soft-preemption comes handy.
"soft-preemption" is described in RFC 5712 https://tools.ietf.org/html/rfc5712 and is useful, well, when You have something to preempt! I.e. when BW of Your links are oversubscribed or are going to be oversubscribed and You have a priority service that uses some of the LSPs then it's time to think about LSP setup priority, hold priority and soft-preeemption on top.
By default reservations are set using Fixed Filter Reservation Style, which means that each session has its own reserved bandwidth. You can see in the example, that if the routers attempt to create the new LSP, also requesting 6Gbps, the new LSP set up will fail. When you configure adaptive, the bandwidth reservation is changed to Shared Explicit, which means the sessions share a single bandwidth reservation. Thus, when you enable adaptive, the new LSP will be successfully created.
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,
I hope this helps !
Regards,AnkurJNCIE x 3 (SP,DC & ENT)Please Mark My Solution Accepted if it Helped, Kudos are Appreciated too!!!