Few Win client machine throws up (only port 1521) ORA-03135 Error

Question

Hi Everyone,
We have been encountering a strange issue with the client --> database connectivity.
Oracle Database Server 11.2.0.3 (64bits) Single Instance - AIX 6.1 (64 bits)
Oracle Client 11.2.0.3 (32 bits) - Windows 2008 R2 Standard (64 bits)
Two windows client machines disconnect from the database server while running a script. The connection hangs when the script is being executed with just the cursor on the screen. After a while, it throws up the ORA-03135- connection lost contact error.
The same script works fine from other windows machine perfectly even on VPN and having different versions of oracle clients installed on them too. Have tried following up a number of KB articles. Have tried setting up sql.net timeout parameters on the server as well as client accordingly. Checked firewall settings with network team. Even tried opening up all firewall ports for test purposes but still the issue persists.
Oracle Support suggested a number of troubleshooting steps and one of the step was the following:
".....Try changing listener port instead of common port 1521. Many routers including Juniper handles Oracle default listener port 1521 specially and thus might impose traffic limit."
Quickly we tried changing the listener port and also the firewall guys changed the port to 1522 from existing 1521. As soon as we did this, we were able to run the scripts fine from both the machines clearly indicating a stable connection. We tried reverting back to 1521 and encountered the same issue.
Anyone aware of this peculiar issue? All help will be really appreciated! Thank you!

Answer 1

Hi Helios,
I have gone through the documents earlier and the respective sql parameters has been set. But again, the problem is ONLY with 2 of our windows client machines and that too when the db listener port is set to default 1521. Our network team confirmed that all the SQL related ALG filtering has been disabled. The Intrusion Protection System (IPS) is also disabled as default between the client - database network VLAN.