4 servers on a site running 7.31 on AIX. Server2 is HDR primary, Server1 secondary. Server3 and Server4 are a spare pair of servers, again in a HDR pair.
Over 100 sessions connected to Server2.

I think I have some sort of DNS or client authentication issue because rebooting server4, kicks the users out of server2 just before server4 comes back to a login. The clients all have setnet32 definitions of the servers specifiying the username of informix and the correct password.

The client machines run a mixture of DNS lookups or flat hosts file to resolve hostnames.

The only way I have found to get around the problem is to bounce Server2 which kills off the now inactive sessions and immediately gets Server2 back on the network i.e. ping/telnet/ilogin start working again.