This article explains how issues can happened during LDP IGP Synchronization
Synchronization between the Label Distribution Protocol (LDP) and the underlying interior gateway protocol (IGP) ensures that LDP is fully established before the IGP path is used for forwarding traffic.
During ldp down event ospf will advertise max metric, quote from ldp-synchronization : "Enable synchronization by advertising the maximum cost metric until LDP is operational on the link."
When ldp session recovers, ospf will advertise normal metric - this event (max metric -> normal metric) will result in route age refresh.
ospf { area 0.0.0.0 { interface ae100.0 { ... ldp-synchronization;
When ldp session goes down ospf (due to "ldp-synchronization") uses max-metric, and when ldp session goes up again ospf metric changed back to normal.
Route age are refreshed on each metric change. Illustration from lab:
root@ILSR1> show route 2.1.1.1/32
Sep 02 10:43:12
...
2.1.1.1/32 *[OSPF/10] 00:12:48, metric 1
> to 10.1.1.2 via ge-0/0/1.0
Sep 2 10:43:54 ILSR1 rpd[73225]: %DAEMON-5-RPD_LDP_SESSIONDOWN: LDP session 2.1.1.1 is down, reason: hold time expired
Sep 02 10:43:56
2.1.1.1/32 *[OSPF/10] 00:00:03, metric 65535
Sep 2 10:44:27 ILSR1 rpd[73225]: %DAEMON-6-RPD_LDP_SESSIONUP: LDP session 2.1.1.1 is up
Sep 02 10:44:44
2.1.1.1/32 *[OSPF/10] 00:00:06, metric 1