Cookie persistence not being maintained if server doesn't send cookie back.

The order of events in your "what we see" description seems unusual. Perhaps we can start by determining the type of cookie persistence are you using - cookie insert, cookie rewrite, or cookie passive? Can you provide a listing of your cookie persistence profile's configuration?

Oh yeah! Sorry. We are using cookie insert with encryption and 3 minutes experation.

These are the parameters: * Method: Insert * Cookie Name: * Timeout: 3 minutes * Encryption: require (it also has a passphrase).

The funny thing is that it looks like persistence is lost sometimes because the F5 is generating a new cookie everynow and then, though the behavior reported happens when the client refreshes the page via a browser (F5).

Do you have "Always Send Cookie" checked? (The default is unchecked.) If unchecked, the BIG-IP system will NOT send the cookie out on each HTTP response. (It's more efficient that way since the cookie is still on the client.) However, since you are specifying an expiration time for the cookie, it means the cookie will expire in 3 minutes, no matter how many times the client sends the cookie to the BIG-IP system on a subsequent HTTP request, as the BIG-IP system never sends an updated cookie with a new expiration time. After 3 minutes, no more persisting; load balance instead, then a new cookie with new 3 minute persistence expiration. If you want the 3 minute expiration to be a sliding 3 minute window - one that gets refreshed on every subsequent HTTP request to the same VIP - then make sure "Always Send Cookie" is checked (enabled).