Symptom
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.
Conditions
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
Workaround
There is no workaround. User can safely ignore the harmless FRR backup paths during the short 'ldp sync not achieved' period.
Further Problem Description
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.
