I am using GNS3 VM and connecting to it remotely from my laptop. I was facing an issue where telnet to GNS3 routers/switches stopped working, though required router or switch was working properly and passing traffic but somehow it was disconnecting the telnet session. This issue happened when I directly close the telnet session, or when my laptop is locked/logged off and upon re-connecting to GNS3 VM, I was unable to take console session to the router/switch until i restart the particular router/switch. Solution to this problem was to properly " exit the console session with cli command " and not closing the putty window directly. Or you can configure your router/switch for console timeout after some time.