Does anyone has any idea what could be considered reasonable in terms of packet drop rate in a FastEthernet interface if we take into account the amount of packets that were forwarded in that interface? For example, 5% of all the packets sent by an interface were dropped. Is this percentage acceptable?
Acceptable drop rate depends on the type of packets, i.e. their protocol and higher applications riding over it. In general, for TCP, I believe a rough rule of thumb is try to avoid more than 1%.
Keep in mind, TCP may try to utilize the full available bandwidth of a path. Drops are often a primary method for it to detect when the path's available bandwidth has been exeeded. I.e., some packet dropping with TCP can be often be expected.
Non-TCP protocols use what they use and are generally non-responsive to dropped packets, with regard to changing their bandwidth usage, although they can be adversly impacted by packet drops. For these protocols, it's often best to try to avoid any drops.