Our Current Setup
We have 4 IIS Servers where our ASP .NET application has been deployed.
In IIS, the session state is configured as SQL Server (External )
All the 4 IIS servers are configured as active pool members in the BIG F5 load balancer
On a normal scenario/traffic, we could see almost equal number of connections established across all the 4 members of the big F5 pool.
If there is an issue with one of the IIS servers, we disable that server from the F5 pool. Even after disabling we don't see the connections are going down in the F5 pool.
Now we will restart that IIS server and the strange thing we have noticed during the restart is: the TCP active connections in the other active servers in the big F5 is drastically increasing (nearly 10 times) and at the same time the CPUs reached out to 100% in all those 3 active servers.
We would need your help to find out what would have caused this issue or if anyone else come across this scenario before.
.NET Framework Version: 4.6
IIS Version: 8.5.9600.16384
SQL Server Version: MS SQL 2016 SP2
Load Balance Algorithm: Round robin
Is the IIS server up and running before adding to the pool? Yes
Any server affinity? SQL Server session management