RSVP-TE Bypass LSP Path SRLG Information Update in Manual and Timer Resignal MBB

This feature enhances procedures of the timer and manual resignal (both delay and lsp options) of the RSVP-TE bypass LSP path by updating the SRLG information of the links of the current path and checking for SRLG disjointness constraint. The following sequence describes the timer and manual resignal enhancements.

CSPF updates the SRLG membership of the current bypass LSP path and checks if the path violates the SRLG constraint of the first primary path that was associated with a PLR of this bypass LSP. This is referred to as the initial Path State Block (initial PSB).
CSPF attempts a new path computation for the bypass LSP using the initial PSB constraints.

MPLS uses the information returned by CSPF and determines if the new bypass path is more optimal.

If SRLG FRR strict disjointness is configured (configure>router>mpls>srlg-frr strict) and CSPF indicates the updated SRLG information of current path violated the SRLG constraint of the PLR of the initial PSB, the new path is more optimal.
Otherwise, MPLS performs additional checks using the PLR of the initial PSB to determine if the new path is more optimal. Table 1 summarizes the possible cases of bypass path optimality determination.

Table 1. Determination of Bypass LSP Path Optimality
PLR SRLG Constraint Check^¹		SRLG FRR Configuration (Strict/Loose)	Path Cumulative Cost Comparison^¹	Path Cumulative SRLG Weight Comparison^¹	More Optimal Path
Current Path	New Path	SRLG FRR Configuration (Strict/Loose)	Path Cumulative Cost Comparison^¹	Path Cumulative SRLG Weight Comparison^¹	More Optimal Path
Disjoint	Disjoint	—	New Cost < Current Cost	—	New
Disjoint	Disjoint	—	New Cost ≥ Current Cost	—	Current
Disjoint	Not Disjoint	—	—	—	Current
Not Disjoint	Not Disjoint	—	—	—	New
Not Disjoint	Not Disjoint	Strict	—	—	Current
Not Disjoint	Not Disjoint	Loose	New Cost < Current Cost	—	New
Not Disjoint	Not Disjoint	Loose	New Cost > Current Cost	—	Current
Not Disjoint	Not Disjoint	Loose	New Cost = Current Cost	New SRLG Weight < Current SRLG Weight	New
Not Disjoint	Not Disjoint	Loose	New Cost = Current Cost	New SRLG Weight ≥ Current SRLG Weight	Current

If the path returned by CSPF is found to be a more optimal bypass path with respect to the PLR of the initial PSB, the following sequence of actions is taken:
- MPLS signals and programs the new path.
- MPLS moves to the new bypass path the PSB associations of all PLRs which evaluation against Table 1 results in the new bypass path being more optimal.
- Among the remaining PLRs, if the updated SRLG information of the current bypass path changed and SRLG FRR loose disjointness is configured (configure>router>mpls>srlg-frr), MPLS keeps this PLR PSB association with the current bypass path.
- Among the remaining PLRs, if the updated SRLG information of the current bypass path changed and SRLG strict disjointness is configured (configure>router>mpls>srlg-frr strict), MPLS evaluates the SRLG constraint of each PLR and performs the following actions.
  1. MPLS keeps with the current bypass path the PSB associations of all PLRs where the SRLG constraint is not violated by the updated SRLG information of the current bypass path.
    
    These PSBs are re-evaluated at the next timer or manual resignal MBB following the same procedure, as described in RSVP-TE Bypass LSP Path SRLG Information Update in Manual and Timer Resignal MBB.
  2. MPLS detaches from current bypass path the PSB associations of all PLRs where the SRLG constraint is violated by the updated SRLG information of the current bypass path.
    
    These orphaned PSBs are re-evaluated by the FRR background task, which checks unprotected PSBs on a regular basis and following the same procedure, as described in RSVP-TE Bypass LSP Path SRLG Information Update in Manual and Timer Resignal MBB.
If the path returned by CSPF is found to be less optimal then the current bypass path or if CSPF did not return a new path, the following actions are performed.
- If the updated SRLG information of the current bypass path did not change, MPLS keeps the current bypass path and the PSB associations of all PLRs.
- If the updated SRLG information of the current bypass path changed and SRLG FRR loose disjointness is configured (configure>router>mpls>srlg-frr), MPLS keeps the current bypass path and the PSB associations of all PLRs.
- If the updated SRLG information of the current bypass path changed and SRLG strict disjointness is configured (configure>router>mpls>srlg-frr strict), MPLS evaluates the SRLG constraint of each PLR and performs the following actions.
  1. MPLS keeps with the current bypass path the PSB associations of all PLRs where the SRLG constraint is not violated by the updated SRLG information of the current bypass path.
    
    These PSBs are re-evaluated at the next timer or manual resignal MBB following the same procedure, as described in RSVP-TE Bypass LSP Path SRLG Information Update in Manual and Timer Resignal MBB.
  2. MPLS detaches from current bypass path the PSB associations of all PLRs where the SRLG constraint is violated by the updated SRLG information of the current bypass path.
    
    These orphaned PSBs are re-evaluated by the FRR background task, which checks unprotected PSBs on a regular basis and following the same procedure, as described in RSVP-TE Bypass LSP Path SRLG Information Update in Manual and Timer Resignal MBB.