When the bypass resignal timer is enabled, MPLS makes a request to CSPF for the best path for each dynamic bypass LSP originated on the node. The constraints of the first associated LSP primary path that originally triggered the signaling of the bypass LSP must be satisfied. In order to do this, MPLS saves the original Path State Block (PSB) of the LSP primary path, even if the path is torn down.
If CSPF returns no path or returns a new path that is equal in cost to the current path, the PSB associations are not updated. If CSPF returns a new path with a different cost from the current one, MPLS signals it.
When the new path is successfully signaled, MPLS evaluates each PSB of each PLR (that is, each unique avoid-node or avoid-link constraint) associated with the older bypass LSP path to check whether the corresponding LSP primary path constraints are still satisfied by the new bypass LSP path. If the constraints are satisfied, the PSB association is moved to the new bypass LSP.
If the constraints are not satisfied, the PSB remains associated with the older bypass LSP and will be checked at the next background PSB re-evaluation or at the next timed or manual bypass reoptimization. Additionally, if the older bypass LSP is SRLG disjoint with a primary path that has the non-strict SRLG condition and the new bypass LSP is not SRLG disjoint, the PSB association is not moved.
If a PLR associated with a bypass LSP is active, the corresponding PSBs remain associated with the older bypass LSP until the global revertive make-before-break (MBB) operation tears down all corresponding primary paths, which also causes the older bypass LSP to be torn down.
When the bypass resignal timer is configured, a PSB re-evaluation task is initiated that runs in the background of each RSVP-TE session to determine whether an existing manual or dynamic bypass is more optimal for that session. If the PSB re-evaluation task finds a more optimal bypass, it moves the PSB association to it. If the PLR for this session is active, no action is taken and the PSB is re-examined at the next re-evaluation.
The periodic bypass reoptimization feature evaluates only the PSBs of the PLRs associated with that bypass LSP and only against the new bypass LSP path. The background re-evaluation task will, however, audit all PSBs on the system against all existing manual and dynamic bypass LSPs. PSBs that have not been moved by the dynamic or manual re-optimization of a bypass LSP, due to the PSB constraints not being met by the new signaled bypass LSP path, will be re-evaluated by the background task against all existing manual and dynamic bypass LSPs.
The background re-evaluation task also checks for PSBs that have requested a node-protect bypass LSP but are currently associated with a link-protect bypass LSP, as well as PSBs that have requested FRR protection and have no association. The background task is in addition to the attempt made when an RESV message is received on the protected LSP path, which ensures the association is completed faster.
This feature is not supported with inter-area dynamic bypass LSPs.