Hello Krzysztof
Ideally when an active sup crashes or is removed, standby sup overtakes all operations immediately without interruptions. Simple view is that there is keepalive between two sups, and when the second sup doesn't receive it - it becomes active. (For example when sup or any linecard is removed - nexus detects ejectors state - and does appropriate actions)
But please consider that depending on N7K configuration and working environment (neighbor configuration - timers, bgp graceful restart etcc) result can be different.
Here is the brief info how chassis with 2 supervisors realize high availability.
Both supervisors have the following:
System manager - special process that watch all the processes running in the sup and through redundancy driver exchanges HA signals with other sup and sync's up.
MTS - maintains communications between applications which are running. Synchronized between sups via special out of band channel.
Persistent storage Services (PSS) - each process saves checkpoint (running/runtime data) into PSS which helps to seamlessly restore each process when it crashes. Published across supervisors and linecards.
Standby sup is always in hot standby mode. Most of the processes on standby sup follows the state their "active" peers.
So you see that there is a big work behind the scene to provide high availability.
But each particular case should be investigated.
Hope that helps,
Alex
