irule not closed with else { }

Nandhi I believe you mean that if the following is configured what will happen. Please note that this only has an if without an else following it.

if { [HTTP::host] == "example.com" }{
    HTTP::redirect "http://www.example.com/"
}

 If the rule mentioned was in place traffic would continue on unless of course it matched the exact string of "example.com" but if it didn't it would be directed to whatever default action the VS has configured such as a pool that would receive traffic from the VS. Typically the else statement is used to force a specific behavior if your traffic does not match the if statement.