Hi,
NSB comes into play during Routing Engine failovers to maintain L2 Control Protocol information on the backup RE:
http://www.juniper.net/techpubs/en_US/junos12.3/topics/concept/nonstop-bridging-ex-series.html
http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/nonstop-bridging-overview.html
Looking at the logs:
Jun 7 01:42:53 sw-cloud-ex3300-stack sfid[1303]: JTASK_SCHED_SLIP_KEVENT: 21 sec 481478 usec kevent block Jun 7 01:42:53 sw-cloud-ex3300-stack chassism[1302]: JTASK_SCHED_SLIP_KEVENT: 21 sec 486749 usec kevent block Jun 7 01:42:53 sw-cloud-ex3300-stack lldpd[10106]: JTASK_SCHED_SLIP: 21 sec scheduler slip, user: 0 sec 0 usec, system: 0 sec, 557 usec Jun 7 01:42:53 sw-cloud-ex3300-stack eswd[10087]: JTASK_SCHED_SLIP_KEVENT: 21 sec 526552 usec kevent block Jun 7 01:42:53 sw-cloud-ex3300-stack eswd[10087]: Root bridge in context 0 changed from 4:cc:4e:24:3a:e9:b8 to 8192:84:b5:9c:46:79:01 Jun 7 01:42:53 sw-cloud-ex3300-stack cfmd[10091]: JTASK_SCHED_SLIP_KEVENT: 23 sec 280174 usec kevent block
You may need to try correlate the LACP flap event with other events that may have triggered this. Maybe check whether there were any RE failovers at that time, in this case VC master switchover.
In the absence of NSB should there be a RE failover, this would cause the L2 Control Protocol process [l2cpd] to restart. Did all the LACP interfaces flap at this time?
As for fast and slow periodic rates, as per this link the rate is adaptable:
The interval can be fast (every second) or slow (every 30 seconds). You can configure different periodic rates on active and passive interfaces. When you configure the active and passive interfaces at different rates, the transmitter honors the receiver’s rate.