When a bypass LSP is adaptive, it allows for modification to some of its parameters while enabled. Without the adaptive capability, the user must disable the bypass LSP before modifying its parameters.
LSP make-before-break (MBB) is a procedure where a new instance of the LSP is brought up, and a switch changes from the current instance to the new instance of the LSP. The new instance of the LSP may have been setup with one or both of the following characteristics:
Modifying any adaptive parameters of an adaptive bypass LSP creates a new instance of the bypass LSP with the updated parameter. Upon automatic or manual commit of changes to the bypass LSP, the new instance of the bypass LSP calculates a new path using newly modified parameters and tries to bring itself up. Once the new instance is up, a make-before-break (MBB) switch takes place from the current instance of the bypass LSP to the new instance.
Below is an example of a bypass LSP re-optimization at an FRR LSP ingress.
Adding the adaptive capability to a bypass LSP enables the bypass LSP to switch to a better route by way of an action known as LSP path re-optimization. The calculation of the re-optimized route can occur on demand based on user request or periodically based on the expiration of a configurable re-optimization timer. Path re-optimization succeeds when a better path becomes available, a new instance of a better path comes up, and an MBB switch takes place from the current instance to the new instance.
An adaptive bypass LSP MBB new instance does not take place if there are any active (traffic carrying or backup in a switched state) backups on the bypass LSP.
When a bypass LSP goes through MBB due to re-optimization, or an adaptive parameter change, all backups using it are torn down from that bypass. After this, the FRR LSP triggers a new backup query. This new query procedure is independent of the previously associated bypass LSP of the backup path. Based on the network topology, availability, and constraint fulfilment, the same old bypass LSP may or may not be chosen.
Below is an example for bypass LSP MBB due to re-optimization or a new instance commit.
