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In an ISIS router with SR, TI-LFA FRR and LDP sync enabled, when 'ldp sync not achieved', one may observe: 1) No FRR Backup path for a primary path of an ECMP group 2) FRR backup path may be installed in other cases 2) is incorrect because before 'ldp sync is achieved', label forwarding and FRR features are not supported and ISIS should not program any FRR backup path.
Relevant config: router isis address-family [ipv4 | ipv6] unicast segment-routing mpls interface address-family [ipv4 | ipv6] unicast fast-reroute per-prefix fast-reroute per-prefix ti-lfa mpls ldp sync To examine LDP sync status: show isis interface | inc LDP MPLS LDP Sync (L1/L2): Disabled/Disabled MPLS LDP Sync (L1/L2): Enabled/Enabled LDPv4 Sync Status: Not Achieved MPLS LDP Sync (L1/L2): Enabled/Enabled LDPv6 Sync Status: Not Achieved
There is no workaround. User can safely ignore the harmless FRR backup paths during the short 'ldp sync not achieved' period.
The 'ldp sync not achieved' should be a short transient period (usually a few sec). If router is stuck at 'ldp sync not achieved', please: - troubleshoot the ldp sessions - if ldp sessions are not required or not running, please remove the 'mpls ldp sync' config.
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