‘Action=jump jump-target=drop’ means that these packets should now be processed through rules with ‘chain=drop’. Do you have any rules with ‘chain=drop’ ?
It is true, I havent any rule with chain=drop. I created one and all seems OK.
For blocking a specific traffic, may I use now chain=drop instead of input, output or forward?
Action=jump can be handy if you want to classify specific traffic and apply more than one rule to it. You should read manual about this: http://wiki.mikrotik.com/wiki/Firewall_filter . Pay most attention to action=jump and action=return.